Effect of Complex Reservoir Geometries and Completion Practices on Production Analysis
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 111285, "Evaluation of the Effect of Complex Reservoir Geometries and Completion Practices on Production Analysis," by S.A. Cox, SPE, R.P. Sutton, SPE, and R.P. Stolz, SPE, Marathon Oil Co.; R.D. Barree, SPE, Barree & Assocs.; and M.W. Conway, SPE, Stim- Lab, prepared for the 2007 SPE Eastern Regional Meeting, Lexington, Kentucky, 17–19 October. The paper has not been peer reviewed.
Complex reservoir geometries can influence the results obtained from transient-production-decline analysis. For tight gas reservoirs, it is common to observe limited drainage areas and linear-flow geometries. In some cases, these results are inconsistent with the expected geological, structural, and depositional character of the reservoir. Complex reservoir geometries and flow conditions such as liquid loading can contribute to this phenomenon. Numerical-simulation cases will be used to generate the control data sets to demonstrate these effects. The full-length paper presents seven cases showing the effects of complicated reservoir flow conditions on the results obtained from production analysis.

Introduction
Production analysis commonly is used to evaluate completion efficiency and effective drainage area of producing wells. The technique is a type-curve-matching technique. Flowing bottomhole pressures and production rates are treated as a long-term production/drawdown test, providing an estimate of effective drain-age area, flow capacity, and effective fracture half-length through type-curve analysis. This technique, applicable for both oil and gas producers, originally was presented for a single-layer reservoir. The technique is applied to layered systems by assuming average properties for the productive zones.
Production-analysis techniques are well suited for the analysis of tight gas reservoirs. Tight gas reservoirs typically stay in transient flow for an extended period of time, and therefore daily pressure and rate data can be used to analyze the transient behavior of the reservoir around the producer. Tight gas wells typically drain a limited area and often experience linear flow for an extended period of time after completion.

Simulation
Numerical-simulation cases were constructed to develop production and pressure profiles used for the analysis. The base case for this analysis was a fracture-stimulated radial-flow model. The reservoir geometry for the single-layer cases was a square, and for the multilayer case, the top layer was a high-permeability channel. Production rate from the models was controlled by a 350-psi constant tubing pressure with a 10,000-Mscf/D maximum rate limit. The base case was run for 2 years.

Base Case
The base case represents a well producing from the center of a 40-acre square reservoir. The results of this case demonstrate the ability of the production-analysis technique to match simulation results. Fig. 1 is the type-curve match for this case. It should be noted that the match assumes an infinite-conductivity fracture and that an acceptable match also was obtained by use of a finite-conductivity model with a 200-ft fracture half-length, Xf, and a 75-md-ft fracture conductivity. Logarithmic gridding was used in the simulation cases to capture the transient flow in the reservoir and eliminate numerical dispersion.