Productivity of Selectively Perforated Horizontal Wells

Abstract

This paper investigates the effect of selective perforating on horizontal well performance. Theoretical investigation is based on a general 3D analytical IPR model that was published previously. For a given perforation design, the changes in flow rate, pseudo-steady state productivity, and cumulative production can be computed using the solution. The investigation shows that the ratio of total perforated 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 due to 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. 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 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. 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 ±10–20 degrees from the vertical. This type of perforating design has been referred as to oriented perforating. Background Perforating has been one of the most common completion methods for different type 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 have been implemented in the horizontal wells drilled in many fields such as Andrew,1 Oseberg,2 Statfjord,3 Elk Hills,4 and others.5–10 Horizontal wells in the Andrew Field were perforated underbalanced to minimize perforating debris and to avoid productivity impairment.1 Variable perforating density and blank sections were used to obtain uniform influx along the wellbore. The weak zones were perforated with 4-spf density and deep penetrating charges. Perforations were oriented 25° on either side of vertical to prevent the perforation collapse. Standard 60° phased guns with 4-spf were used to perforate the stable sands.

Challenges in Completing Long Horizontal Wells Selectively

“…Furthermore, a penetration ratio (perforated length/lined length) above 20 to 30% provides diminishing returns in comparison to an openhole well (Yildez 2006). Furthermore, a penetration ratio (perforated length/lined length) above 20 to 30% provides diminishing returns in comparison to an openhole well (Yildez 2006).…”