Pressure Transient Testing In High Transmissibility Reservoirs: Design :mnd Analysis Considerations
Authors: Kabir C.S. , Willmon J.H. 
Published: March 2010

Introduction

An analysis of a large number a/well tests in east-central and southeastern Alberta indicates that the pressure transient data normally obtained from the shallow wells (&lt;1000 m) in high· permeability (&gt; 300 mD) gas or oil reservoirs are not amenable to proper semilog analysis. The problem is an apparent lack of true infinite reservoir behaviour for sufficient time to permit semilog analysis either in buildup or drawdown situations. The situation occurs when the semilog line indicates zero slope, preventing any analysis due to very fast buildup or stabilization of pressure. Consequently, the reservoir properties of flow capacity (kh), skin (s) and inertial turbulent flow factor (D) cannot be estimated.

This paper examines the reasons for the lack of analyzable infinite-acting transient data obtained from wells in high-transmissibility reservoirs. It is demonstrated with the aid of generalized plots that by increasing the drawdown at the sandface and making a proper choice of pressure recorder one may obtain enough transient data for determining the measurable semilog slope. Thus, proper well tests can be designed for estimating useful in-situ properties of kh, S and D. The ability to obtain measurable semilog slopes is, however, dictated by the reservoir transmissibility.

Introduction

The proper design of buildup and drawdown tests conducted on wells characterized by high permeability is essential to the generation of analyzable data. Experience with well test data from the Mannville zones in southeastern and east-central Alberta has shown that conventional semilog analysis techniques are frequently not applicable or yield misleading estimates of formation properties. Design calculations can provide insight as to the ability to overcome this problem by increasing the magnitude of the pressure response, and will indicate pressure gauge sensitivity requirements.

The rapid stabilization of pressure encountered in a well in a typical high-transmissibility reservoir is demonstrated in Figure 1. The observed transient data are not amenable to conventional semilog analysis because the pressure response cannot be measured due to the sensitivity of the pressure gauge which was used. The ability of wells to stablize quickly does not pose a problem for estimates of stabilized formation pressures or deliverability. However, there appears to be no technique for obtaining in-situ measurements of flow capacity (kh), physical skin (S) or, in the case of gas reservoirs, the inertial-turbulent flow factor (D). The ability to increase the pressure response to within gauge sensitivity to facilitate conventional analysis is desirable to provide reliable information about in-situ reservoir properties. The determination of accurate values of basic reservoir properties is valuable for reservoir studies and as an aid to determining stimulation potential. The kh values obtained can be used in determining productivity indices of each well in a given reservoir. These values of productivity indices can be plotted against individual flow capacity to determine which wells may benefit from stimulation. The benefits associated with any given percentage increase in production are far greater for wells intercepting high-transmissibility reservoirs than wells in lower-transmissibility reservoirs.