Inflow Performance Relationship for Perforated Horizontal Wells
- Turhan Yildiz (Colorado School of Mines)
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- September 2004
- Document Type
- Journal Paper
- 265 - 279
- 2004. Society of Petroleum Engineers
- 5.6.4 Drillstem/Well Testing, 5.3.2 Multiphase Flow, 3 Production and Well Operations, 3.2.5 Produced Sand / Solids Management and Control, 5.6.8 Well Performance Monitoring, Inflow Performance, 5.1.1 Exploration, Development, Structural Geology, 1.8 Formation Damage, 2.4.5 Gravel pack design & evaluation, 2.4.3 Sand/Solids Control, 1.6 Drilling Operations, 2.2.2 Perforating, 5.7.2 Recovery Factors
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This paper presents an investigation of perforated horizontal-well performance in areally anisotropic reservoirs. Theoretical investigation is based on a 3D analytical IPR model. The analytical IPR model considers an arbitrary distribution of perforations along the completed segments. Changes in flow rate, pseudosteady-state productivity, and cumulative production can be computed using the solution.
The analytical IPR model was compared with the models available in the literature and verified. It was then used to investigate the effects of well and reservoir parameters on the inflow performance of perforated horizontal wells.
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 producing multiple zones with large productivity contrast effectively.
Open-completed horizontal wells with negligible wellbore pressure loss display a u-shaped influx profile; fluid velocities at the heel and toe end of the well are higher than those at the midsection of the well. Several simulation studies have shown that water/gas prematurely breaks through at the heel end of the well and causes inefficient sweep.
Uniform influx along the horizontal wellbore is desirable to delay premature water/gas breakthrough and improve the sweep efficiency. Water/gas breakthrough could be delayed by restricting the flow and communication between reservoir and wellbore at the intervals where local fluid velocities are higher.
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, partial completion and enforcing uniform inflow along the wellbore by variable shot density reduce the well productivity. Therefore, a complete engineering analysis is required to weigh the gains from improved sweep provided by uniform inflow against the loss in well productivity.
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 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. Solid 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.
Perforating has been one of the most common completion methods for vertical wells requiring sand control and prevention of wellbore collapse and water/gas intrusion. The performance of perforated vertical wells has been investigated extensively. However, only a few modeling studies have dealt with the productivity of perforated horizontal wells. Field applications of the perforated horizontal wells have preceded the modeling efforts. Some of the field applications and research studies are summarized below to acknowledge the previous contribution.
Horizontal wells are mostly completed openhole with slotted liners or prepacked screens. However, in some fields, horizontal wells may need to be cased and perforated. Selective perforating has been implemented in the horizontal wells drilled in many fields such as Bongkot,1 Joanne,2 Andrew,3 Oseberg,4,5 Wytch Farm,6 Statfjord,7 Elk Hills,8 Beryl,9 Dan,10 Alba,11 Yowlumne,12 and Prudhoe Bay.13 King14 pointed out the significance of gun clearance and positioning in perforating and gravel packing the horizontal wells.
Previous Modeling Studies.
The performance of perforated vertical wells has been the subject of many modeling studies. 15-19 Perforation shot density and length and penetration ratio are found to be the dominant parameters controlling the flow efficiency of vertical wells.
Inflow performance models for horizontal open holes are relatively simple and well known.20-22 However, these models ignore the near-wellbore completion effects and consider that the total drilled horizontal length is open to flow. In many cases, horizontal wells may be completed at selected intervals along the well axis. The horizontal wells partially completed at multiple producing segments are referred as selectively completed wells. Several modeling studies on the performance and pressure behavior of selectively-completed horizontal wells have appeared in the literature.23-26 Although these models account for selective completion effect, they all ignore the details of flow convergence because of the existence of perforations and slots in the near-wellbore region.
A few studies have addressed how the perforations and slots influence the flow into horizontal wells.27-34 Landman and Goldthorpe27 and Marett and Landman28 presented steady-state models for flow into perforations distributed around a horizontal wellbore. They also proposed the use of variable perforation shot density to obtain a uniform influx along the wellbore. Gonzalez-Guevara and Camacho-Velazques29 extended the model of Refs. 27 and 28 to account for multiphase flow in the wellbore and reservoir and non-Darcy flow around the perforations.
Asheim and Oudeman30 presented a simple algorithm to predict the perforation shot density yielding uniform influx along a horizontal well. They concluded that optimal perforation distribution is rate-dependent, and enforcing uniform flux along the wellbore reduces the well productivity/injectivity.
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