This paper presents a comprehensive study on the productivity and flow efficiency of horizontal wells completed with slotted-liners or perforations. The study is based on a semi-analytical model that couples the flow equations in the reservoir and wellbore. The reservoir model takes into account the 3D convergence of flow around perforations and slots. The wellbore flow model considers the pressure losses inside the horizontal well and the effect of axial influx at the perforations and slots. A new, experimental apparent friction factor correlation is used for horizontal wellbore flow computations with perforations and slots. The model is capable of incorporating the effects of selective completion and non-uniform skin distribution. The results of this study indicate that software based on detailed semi-analytical models can provide a powerful tool to design, predict, and optimize horizontal well completions. It is also shown that horizontal wells deserve genuine guidelines to optimize their completions. For example, horizontal wells are shown to require significantly lower slot and perforation densities to accomplish optimum PI compare to vertical wells. Similarly, in horizontal wells, the effect of slot or perforation phasing becomes more important as the anisotropy of the formation increases. Introduction Horizontal wells are one of the most important strategic tools in petroleum exploitation.1 As a result of the advances in drilling and completion technologies in the last two decades, the efficiency and economy of horizontal wells have significantly increased. Today, horizontal well technology is applied more often and in many different types of formations. The state of the art applications of horizontal well technology require better completion designs to optimize production rates, long-term economics, and ultimate producible reserves. Horizontal well completions can be categorized as natural completion, sand-control completion, and stimulation completion. Natural completion includes open-hole, slotted-linear, and cased and perforated completions. Sand-screens, prepacked screens, and gravel packing are the completions used for sand-control. Stimulation completion includes completion with hydraulic fracturing and fracturing with gravel packing (fracpack or stimpack). All of these completion methods have been used in practice under different reservoir conditions.2,3,4 In a horizontal well, depending upon the completion method, fluid may enter the wellbore at various locations and at various rates along the well length. Fig. 1 illustrates the interplay between the pressure and flux distribution along the wellbore through the completion openings. The complex interaction between the wellbore hydraulics and reservoir flow performance depends strongly on the distribution of influx along the well surface and it determines the overall productivity of the well. Therefore, the optimization of well completion to improve the performance of horizontal wells is a complex but very practical and important problem. The complexities of the numerical simulation of horizontal well completions make analytical models extremely attractive. However, the inherent difficulties of the analytical solutions caused by the complex flow geometries, excessive number of perforations or slots, and non-uniform distribution of flux along the horizontal well calls for the challenging task of developing efficient computational algorithms.

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In this study, the available methods and software to predict the well productivity and total skin factor in fully perforated vertical wells have been reviewed. The methods have been compared against the experimental data obtained on an electrolytic apparatus, and their accuracy has been investigated. It has been observed that the 3D semianalytical model, SPAN 6.0 software, and the simple hybrid model described in this paper replicate the experimental results very well. On the other hand, the results estimated from the McLeod method and the Karakas-Tariq method substantially deviate from the experimental data; hence, these models/methods should be used with caution.The literature hosts many equations to predict the total skin factor in partially perforated vertical wells. Some of the available models have been tested against the results from the 3D semianalytical model. It has been shown that total skin-factor equations based on the summation of individual components do not work. The 3D semianalytical model has been modified to build an approximate model for fully and partially perforated inclined wells in isotropic formations. Additionally, a simple hybrid model to compute total skin factor in perforated inclined wells has been presented. The hybrid model for perforated inclined wells agrees well with the approximate 3D model. Some of the available models to calculate total skin factor in perforated inclined wells have been compared to the approximate 3D model, and their accuracy has been discussed.Finally, a simple model to predict total skin factors in perforated horizontal wells has been developed. The application using the simple model has demonstrated that a combination of long wellbore length and perforations bypassing the damaged zone could overcome the destructive effect of severe formation damage around the wellbore.

This paper was prepared for presentation at the 1999 SPE Annual Technical Conference and Exhibition held in Houston, Texas, 3–6 October 1999.

This paper presents a comprehensive investigation of perforated horizontal well performance in areally anisotropic reservoirs. Theoretical investigation is based on a general 3D analytical IPR model. The analytical IPR model considers an arbitrary orientation and distribution of perforations along the completed segments. Changes in flow rate, pseudo-steady 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 partially perforated horizontal wells. Introduction Horizontal wells may be perforated in selected intervals due to several reasons. The most common reasons for selective completion are cost effectiveness, delaying premature water/gas breakthrough, preventation of 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 mid section of the well. Several simulation studies have shown that water/gas prematurely breaks through at the heel end of the well and results an 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 weight 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. Liner and 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. Background Perforating has been one of the most common completion methods for vertical wells requiring sand control and preventation of wellbore collapse and water/gas intrusion. The performance of perforated vertical wells has been investigated extensively. However, only a few modeling studies has dealth 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.