SummaryThis paper investigates the effect of selective perforating on horizontalwell performance. Theoretical investigation is based on a general 3D analytical model that was published previously. For a given perforation design, the changes in flow rate, pseudosteadystate productivity, and cumulative production can be computed using the solution. The investigation shows that the ratio of totalperforated length to the drilledwell length is the most dominant parameter controlling the longterm performance of the selectively perforated horizontal wells. The other important parameters are the degree of formation and perforating damage. We additionally examined the effect of the socalled “oriented perforating” on the horizontalwell performance in isotropic andanisotropic formations. Our research shows that accurately oriented perforating could significantly improve the well productivity in anisotropic formations.
Productivity of Selectively Perforated Horizontal Wells Turhan Yildiz, SPE, Colorado School of Mines Summary This paper investigates the effect of selective perforating on horizontal-well performance Theoretical investigation is based on a general 3D analytical model that was published previously For a given perforation design, the changes in flow rate, pseudosteadystate productivity, and cumulative production can be computed using the solution The investigation shows that the ratio of totalperforated length to the drilled-well length is the most dominant parameter controlling the long-term performance of the selectively perforated horizontal wells The other important parameters are the degree of formation and perforating damage We additionally examined the effect of the so-called “oriented perforating” on the horizontal-well performance in isotropic and anisotropic formations Our research shows that accurately oriented perforating could significantly improve the well productivity in anisotropic formations Introduction Selective Perforating Horizontal wells may be perforated in selected intervals for several reasons The most common reasons for selective completion are reducing the cost, delaying premature water/gas breakthrough, preventing wellbore collapse in unstable formations, and effectively producing multiple zones with large productivity contrast Selective perforating with blank sections provides flexibility for future intervention and workover options and for shutting off the sections subject to excessive water/gas intrusion On the other hand, selective completion could hurt the well productivity Oriented Perforating The orientation of perforations is also a concern in optimizing well productivity Perforations aligned with minimum stress direction produce more sand To reduce the risk of sand production, it may be better to orient the perforations vertically Additionally, subsurface rocks exhibit horizontal permeabilities that are higher than vertical permeabilities Therefore, perforation tunnels perpendicular to higher permeability would possess better flow efficiency On the other hand, debris resulting from the perforation process has to be surged out of the tunnels to improve the productivity of the perforated completions It is more difficult to clean the perforations on the low side of the horizontal wells Liner and solids debris in the low-side perforation tunnels may not be removed under the typical underbalance pressures applied Vertically oriented perforation tunnels at the top side of the horizontal wellbore are preferred for better perforation stability and cleanup efficiency However, if the perforations are to be packed, it is difficult to transport the gravel into vertically oriented tunnels at the top side Field observations and sand-production models have shown that the stability of the perforation cavity may be weakened if all the perforations are oriented vertically with a phasing angle of zero Therefore, to minimize the sand production and to create more stable perforations, it may be better to orient the perforations ±10 to 20° from the vertical This type of perforating design has been referred to as oriented perforating Copyright © 2006 Society of Petroleum Engineers This paper (SPE 90580) was first presented at the 2004 SPE Annual Technical Conference and Exhibition, Houston, 26–29 September 2004 Original manuscript received for review 08 June 2004 Paper peer approved 28 April 2005 February 2006 SPE Production & Operations Background Perforating has been one of the most common completion methods for different types of wells requiring sand control Perforating may also be needed to prevent wellbore collapse and to delay the production of unwanted fluids such as water and gas Selective perforating has been implemented in horizontal wells drilled in many fields such as Andrew (Kusaka 1998), Oseberg (Sognesand et al 1994), Statfjord (Kostøl and Østvang 1995), Elk Hills (Gangle 1995), and others (Benavides et al 2003; Stenhaug et al 2003; Sulbaran et al 1999; Eriksen et al 2001; Tronvoll et al 2004; Hillestad et al 2004) Horizontal wells in the Andrew field were perforated underbalanced to minimize perforating debris and to avoid productivity impairment (Kusaka et al 1999) Variable perforating density and blank sections were used to obtain uniform influx along the wellbore The weak zones were perforated with 4-shots per ft (spf) density and deep-penetrating charges Perforations were oriented 25° on either side of the vertical to prevent the perforation collapse Standard 60°-phased guns with spf were used to perforate the stable sands Sognesand et al (1994) described the use of partial perforation schemes to obtain uniform inflow along the horizontal and multilateral wells in the Oseberg field 45 or 90°-phased guns were used The perforation density was mostly spf Production logs run in the Oseberg horizontal wells showed that the inflow profile could be controlled by selective perforations; however, Sognesand et al (1994) reported significant reduction in well productivity because of selective perforating Extended-reach and horizontal wells in the Statfjord field were designed to penetrate several zones (Kostøl and Østang 1995) Most of the wells were perforated with 12 spf Production logs indicated plugged perforations across the sections perforated at overbalanced conditions Gangle (1995) reported the use of horizontal wells intersecting several steeply dipping beds in the Elk Hills field The wells were selectively perforated at multiple zones The theory, field applications, equipment, and operational procedures for oriented perforating are all described in Kusaka et al (1998), Benavides et al (2003), Stenhaug et al (2003), Sulbaran et al (1999), Eriksen et al (2001), Tronvoll et al (2004), and Hillestad et al (2004) Oriented perforating has been successfully implemented in an unidentified field in the North Sea (Benavides et al 2003), the Visumd field (Stenhaug et al 2003), Eocene C reservoir in Lake Maracaibo (Sulbaran et al 1999), the Varg field (Eriksen et al 2001; Tronvol et al 2004), and the Norne field (Hillestad et al 2004) The performance of perforated wells has been investigated extensively However, the majority of the studies have concentrated on vertical wells and only a few studies have dealt with the productivity of perforated horizontal wells A summary of productivity models for perforated vertical wells may be found in Bell et al (1995) and Karakas and Turiq (1991) Inflow-performance models for horizontal openholes are relatively simple and well known In some cases, horizontal wells may be completed/perforated at selected intervals Several modeling studies on the performance of selectively completed and selectively perforated horizontal wells have appeared in the literature (Goode and Wilkinson 1991; Retananto et al 1997; Yildiz 2004; Marett and Landman 1993; Thomas et al 1998; Göktas¸ and Ertekin 2000; Tang et al 2001) The selectively completed well 75 Fig 2—SCHW model Fig 1—The decoupled model for perforated horizontal wells models account for the partial-completion effect; however, they ignore the details of flow convergence caused by perforations Several studies have addressed how the perforations influence the flow into horizontal wells (Yildiz 2004; Marett and Landman 1993; Thomas et al 1998; Göktas¸ and Ertekin 2000; Tang et al 2001) Marett and Landman (1993) used a steady-state model to predict the pressure drop in a perforated horizontal well and proposed use of variable perforation-shot density to enforce a uniform influx along the well axis Thomas et al (1998) incorporated the near-wellbore skin term and non-Darcy-flow coefficient into their reservoir simulator and the existing semianalytical models for horizontal wells Although the numerical model allows selective completion of multiple segments, the modified analytical models consider only one single completed segment Recently, Göktas¸ and Ertekin (2000) developed a numerical simulator for perforated horizontal wells The convergent flow into perforations, in the damaged zone and in the crushed zone around perforations, is all accounted for Tang et al (2001) investigated the impact of perforation parameters on horizontal-well performance They observed that the perforation densities higher than 0.5 spf yield a marginal increase in well productivity The objectives of the present study are: (1) to investigate the effect of selective perforating on the long-term well performance and (2) to scrutinize the impact of oriented perforating on well productivity segment A variable local skin around each segment is also incorporated into the SCHW model The additional pressure change caused by perforations is superimposed on the SCHW model in terms of local skin A schematic of a reservoir model for a multisegment horizontal well is given in Fig Model for Perforation Total Pseudoskin (PTP) The PTP model accounts for the flow into perforation tunnels, flow in the damaged zone around the wellbore, and flow across the crushed/compacted zone around the tunnels A schematic of the PTP model is displayed in Fig The perforation pseudoskin is calculated for only a unit formation thickness (e.g., 1-ft thickness); therefore, the PTP model is computationally very efficient The additional pressure caused by perforating, formation damage, and rock crushing around each perforated segment is incorporated into the solution using the expression below: ⌬psj = 141.2 qj Bo ͌kzky Lsj sptVj (1) If the permeability anisotropy is accounted for in the calculation of perforation pseudoskin then, in Eq 1, (kzky)1/2סk should be set The decoupled model was compared against the existing models in the literature and verified The details of the comparison and verification are described in Yildiz (2004) Discussion In this section, we simulate different perforation schemes and discuss the impact of selective perforating and other perforation parameters on rate decline, cumulative production, and productivity Flow Model for Selectively Perforated Horizontal Wells Previously, we presented an analytical model to simulate the transient flow into selectively perforated wells The details of the model development are provided in Yildiz (2004); only a brief description of the model will be given here The 3D flow into a perforated horizontal well is decomposed into two smaller subproblems: a transient 3D model for flow into a selectively completed horizontal well and a perforation totalpseudoskin model for accounting for the flow convergence around the tunnels in the near-wellbore region This model will be referred to as the “decoupled” model The modeling concept is illustrated in Fig Selectively Completed Horizontal-Well (SCHW) Model A multisegment horizontal well in a rectangular parallel-piped reservoir with impermeable external boundaries is considered Multisegmentation allows us to account for the local changes around the wellbore The SCHW model assumes that the completed intervals are fully open to flow all around the perimeter of the 76 Fig 3—PTP model February 2006 SPE Production & Operations during the boundary-dominated-flow period The data set given in Table is used in the simulations Selective Perforating To investigate the impact of selective perforating on the well productivity, we have considered pessimistic, reasonable, and optimistic combinations of the perforation parameters and compared the responses of the perforated wells to the open-completed well in terms of rate decline, cumulative production, and productivity index during boundary-dominated flow The perforation parameters for all the perforated-well cases are given in Table Formation damage and perforation damage were ignored in the comparison Using the listed perforation variables, we calculated the PTP values for each case The PTP values for the perforated-well cases are listed in Table The pessimistic, reasonable, and optimistic perforating scenarios yield PTP values of 13.7, 0.6, and –1.7, respectively For all the cases, we considered 2, 3, 4, and segments and penetration ratios of 20, 40, 60, 80, and 100% In each case, the perforated/completed segments of equal length were symmetrically distributed along the drilled-well axis Additionally, we considered that the well produces at a specified constant wellbore pressure in any given time interval; however, the wellbore pressure drop varies with time in a staircase fashion The wellbore pressure drop is 750 psi in the first year and 250 psi annually in years through 10 The simulated results, given in Figs through and Tables through 6, are compared in terms of rate decline, cumulative production, and productivity ratio The results shown in Figs through are all for the cases with five completed segments The results for other segment numbers display similar characteristics For the data set considered in Tables and 2, the number of completed segments has a negligible impact on transient-rate decline, cumulative production, and the productivity index during boundary-dominated flow Fig 4—The effect of selective completion on the rate response (selective completion, no perforations, sptV=0, five segments) February 2006 SPE Production & Operations Fig shows the results in terms of the ratios of the transient rate of selectively completed wells with five segments (no perforation, zero skin factor) to that of an open hole As can be observed in Fig 5, compared to an open hole, the wells with low penetration ratios (20 to 40%) produce at significantly lower rates at early times However, with time, the completions with a low penetration ratio catch up with the openhole completion The completions with a high penetration ratio (60 to 80%) produce at higher flow rates at early times and produce at approximately the same rate at intermediate times Fig displays the results in terms of the cumulative production It is observed that the completions with high penetration ratios produce almost the same amount of oil as the open hole does at the end of the first year The productivity ratios for the selectively completed cases are presented in Table The productivity ratio is defined as the ratio of the productivity index of the perforated/completed well to that of the open hole Table demonstrates that, for all the penetration ratios considered, the effect of the number of the completed segments is insignificant However, the penetration ratio has a substantial impact on the completed-well response For the case with five completed segments, the productivity ratios for the penetration ratios of 20, 40, 60, and 80% are 0.6, 0.8, 0.91, and 0.98, respectively The results for the pessimistic perforation-design case are summarized in Table and in Figs and It should be recalled that the pessimistic perforation-design case possesses a PTP of sptVס13.7 Figs and illustrate that this magnitude of perforation skin significantly decreases the transient rate and cumulative production from even the well completely perforated (penetration ratio is 100%) Also, Table shows that the pessimistic perforation design drastically hurts the productivity ratios of the perforated wells It can be observed that the number of perforated segments has a negligible effect on the productivity ratio As before, the increasing penetration ratio improves the well productivity hugely The productivity ratios for the pessimistic perforation design are 1.7 to 2.8 times lower than the previous case with no skin factor The PTP factor for the wells with reasonable perforation design was estimated to be quite small: sptVס0.6 Therefore, the performances of the wells having reasonable perforation design were Fig 5—The effect of selective completion on the cumulative production (selective completion, no perforations, sptV=0, five segments) 77 Fig 6—The effect of selective perforating on the rate response of pessimistically perforated well (sptV=13.7, five segments) quite similar to those of the selectively completed wells previously discussed Hence, we only present the productivity ratios of these wells in Table A comparison of the results tabulated in Tables and demonstrates that selectively completed wells with a skin factor of zero perform only slightly better than the wells with reasonable perforation design When deeply penetrating perforation tunnels are created and the formation- and perforating-based damage is minimized, the perforating process may result in a negative PTP factor An optimistic combination of perforation parameters of spfס16, Lpס24 in., dpס0.3 in., and pס90º produces a negative skin of sptV–ס1.7 The simulated responses of the wells perforated with such an optimistic design are displayed in Figs and and in Table The results show that a PTP factor of sptV–ס1.7 enables the perforated wells to perform better than the selectively completed counterparts Under such perforating conditions, even the well with a 40% penetration ratio possesses very good productivity If a small stimulation on the order of sptV–ס1.7 is executed, only 60% of the well needs to be perforated to attain the performance of the ideal open hole Oriented Perforating It should be expected that the performance in wells treated with oriented perforating would be influenced by the orientation of the perforations and the formation anisotropy To investigate the impact of oriented perforating in anisotropic formations, we considered spfס1 and 2, Lpס12 in., and dpס0.2 in Then, we calculated the perforation pseudoskin as a function of formation anisotropy for several different perforation orientations The orientation cases include (1) all vertical perforations with 0° phasing, (2) all vertical perforations with 180° phasing, (3) all horizontal perforations with 0° phasing, (4) all horizontal perfora- Fig 8—The effect of selective perforating on the rate response of optimistically perforated well (sptV=–1.7, five segments) 78 Fig 7—The effect of selective perforating on the cumulative production from pessimistically perforated well (sptV=13.7, five segments) tions with 180° phasing, (5) half-horizontal and half-vertical perforations with 90° phasing, (6) vertical and nearly vertical perforations with 0°/10° phasing, (7) vertical and nearly vertical perforations with 0°/20° phasing, and (8) nearly vertical perforations with 10°/350° phasing The calculated perforation pseudoskins for all the cases are shown in Figs 10 and 11, which are for spfס2 and spfס1, respectively In all the cases, the second case with all vertical perforations with a phasing angle of 180° results in the lowest pseudoskin value Case 3, with all horizontal perforations with 0° phasing, exhibits the highest pseudoskin value Case and Cases through yield almost the same value of perforation pseudoskin Therefore, it can be stated that small deviations, such as 10 to 20°, from the vertical direction not hurt the well productivity in any appreciable manner; hence, orienting the perforation with 0°/10°, 0°/20°, and 10°/350° for sand-management purposes is a safe process The results in Figs 10 and 11 also demonstrate that, as a function of increasing formation anisotropy (kz/kx decreasing), perforation pseudoskin decreases for the orientations represented by Cases and and Cases through Conversely, for Cases and 4, perforation pseudoskin increase with an increase in formation anisotropy Hence, it can be stated that oriented perforating is actually a good practice in highly anisotropic formations Nomenclature Bo סformation volume factor, res bbl/STB ct סtotal compressibility, psi–1 dp סperforation diameter Fig 9—The effect of selective perforating on the cumulative production from optimistically perforated well (sptV=–1.7, five segments) February 2006 SPE Production & Operations h Jfp Joh Jsc Jsp k Lh Lp Ls Lsj Npoh Npsc ס ס ס ס ס ס ס ס ס ס ס ס Npsp ס pi qoh qsc qsp rw sptV sptVj xe xw xs ye yw z zw ⌬psj ס ס ס ס ס ס ס ס ס ס ס ס ס ס ס ס p ס ס height, ft productivity index of fully perforated well productivity index of open-completed well productivity index of selectively completed well productivity index of selectively perforated well permeability, md horizontal-well length perforation length segment length length of the jth segment cumulative production in an open hole, STB cumulative production in a selectively completed well, STB cumulative production in a selectively perforated well, STB initial reservoir pressure, psi flow rate in an open hole, STB/D flow rate in a selectively completed well, STB/D flow rate in a selectively perforated well, STB/D wellbore radius, ft perforation total pseudoskin for vertical wells perforation total skin at the jth segment Length of the reservoir in x-direction Location of the segment tip in x-direction Location of the segment center in x-direction Width of the reservoir in y-direction Location of the well in y-direction vertical direction, ft location of the well in vertical plane, ft additional pressure change caused by perforation total skin porosity angle between the perforation and principal permeability direction viscosity, cp Subscripts s w x y z ס ס ס ס ס segment wellbore x-direction y-direction vertical direction February 2006 SPE Production & Operations References Bell, W.T., Sukup, R.A., and Tariq, S.M.: Perforating, Monograph Series, SPE, Richardson, Texas (1995) 16, 56–73 Benavides, P.S et al.: “Advances in Horizontal-Oriented Perforating Optimize Perforation Efficiency and Production While Maintaining Borehole Stability,” paper SPE 80929 presented at the 2003 SPE Production and Operations Symposium, Oklahoma City, Oklahoma, 22–25 March Eriksen, J.H et al.: “Orienting Live Well Perforating Technique Provides Innovative Sand-Control Method in the North Sea,” SPEDC (September 2001) 16, No 3, 164 Gangle, F.J.: “Improved Oil Recovery Using Horizontal Wells at Elk Hills, California,” SPEDC (March 1995) 10, No 1, 27 Göktas¸, B and Ertekin, T.: “Performances of Openhole Completed and Cased Horizontal/Undulating Wells in Thin-Bedded, Tight Sand Gas Reservoirs,” paper SPE 65619 presented at the 2000 SPE Eastern Regional Meeting, Morgantown, West Virginia, 17–19 October Goode, P.A and Wilkinson, D.J.: “Inflow Performance of Partially Open Horizontal Wells,” JPT (August 1991) 983 Hillestad, E et al.: “Novel Perforating System Used in North Sea Results in Improved Perforation for Sand Management Strategy,” paper SPE 86540 presented at the 2004 SPE International Symposium and Exhibition on Formation Damage Control, Lafayette, Louisiana, 18– 20 February Karakas, M and Tariq, S.M.: “Semianalytical Productivity Models for Perforated Completions,” SPEPE (February 1991) 73 Kostøl, P and Østvang K.: “Completion and Workover of Horizontal and Extended-Reach Wells in the Statfjord Field,” SPEDC (December 1995) 10, No.4, 211 Kusaka, K et al.: “Underbalance Perforation in Long Horizontal Wells in the Andrew Field,” SPEDC (June 1998) 13, No 2, 73 Marett, B.P and Landman, M.J.: “Optimal Perforation Design for Horizontal Wells in Reservoirs With Boundaries,” paper SPE 25366 presented at the 1993 SPE Asia Pacific Oil and Gas Conference, Singapore, 8–10 February Retnanto, A et al.: “Optimization of the Performance of Partially Completed Horizontal Wells,” paper SPE 37492 presented at the 1997 SPE Production Operations Symposium, Oklahoma City, Oklahoma, 9–11 March Sognesand, S., Skotner, P., and Hauge, J.: “Use of Partial Perforations in Oseberg Horizontal Wells,” paper SPE 28569 presented at the 1994 SPE Annual Technical Conference and Exhibition, New Orleans, 25– 28 September Stenhaug, M et al.: “A Step Change in Perforating Technology Improves Productivity of Horizontal Wells in the North Sea,” paper SPE 84910 79 Fig 10—The effect of formation anisotropy on the perforation pseudoskin resulting from oriented perforating (spf=2) presented at the 2003 SPE International Improved Oil Recovery Conference in Asia Pacific, Kuala Lumpur, 20–21 October Sulbaran, A.L., Carbonell, R.S., and Lopez-de-Cardenas, J.E.: “Oriented Perforating for Sand Preventation,” paper SPE 57954 presented at the 1999 SPE European Formation Damage Conference, The Hague, 31 May–1 June Tang, Y et al.: “Performance of Horizontal Wells Completed with Slotted Liners and Perforations,” paper SPE/PS-CIM 65516 presented 2001 SPE/CIM International Conference on Horizontal Well Technology, Calgary, 6–8 November Thomas, L.K et al.: “Horizontal Well IPR Calculations,” SPEREE (October 1998) 1, No 5, 392 Tronvoll, J et al.: “The Effect of Oriented Perforations as a Sand Control Method: A Field Case Study from the Varg Field, North Sea,” paper SPE 86470 presented at the 2004 SPE International Symposium and Exhibition on Formation Damage Control, Lafayette, Louisiana, 18–20 February Yildiz, T.: “Inflow Performance Relationship for Perforated Horizontal Wells,” SPEJ (September 2004) 9, No 3, 265 80 Fig 11—The effect of formation anisotropy on the perforation pseudoskin resulting from oriented perforating (spf=1) SI Metric Conversion Factors cp × 1.0* E–03 סPa·s ft × 3.048* E–01 סm in × 2.54* E+00 סcm psi × 6.894 757 E+00 סkPa spf × 3.28* E+00 סspm *Conversion factor is exact Turhan Yildiz is an associate professor in the Petroleum Engineering Dept at Colorado School of Mines, Golden, Colorado (e-mail: tyildiz@mines.edu) Previously, he worked for U of Tulsa, Simulation Sciences, Istanbul Technical U., and Louisiana State U Currently, he is involved in modeling of complex reservoir flow problems and intelligent/multilateral well design Yildiz holds a BS degree from Istanbul Technical U and MS and PhD degrees from Louisiana State U., all in petroleum engineering He serves on SPE Editorial Committee February 2006 SPE Production & Operations