Abstract

Well test analyses are normally performed using a combination of type-cun1e and semi-log analysis. Unfortunately, calculated values of reservoir rock properties such as permeability (transmissivity) or near wellbore damage can vary by a significant amount between the two methods, Unless a history match is conducted, the best value to use is ambiguous.

A practical approach is introduced in this paper which requires a single solution to two or more well test analysis methods, This is accomplished by coupling the multiple solution parameters to derive a testing criterion for a unique solution, i.e. first combining two equations into one, then using this single formulation to determine whether the analyzed data points meet certain requirements simultaneously between the two or more solutions. It is assumed that initial boundary conditions used in deriving the previously separate solutions are the same or very similar.

Two examples are presented, one from a high transmissivity groundwater flow (water source) and one from a low permeability oil sands reservoir. The purpose is to demonstrate the universality of the method and show the actual change in results. For example, a previously reported value of formation compressibility was changed by 100 percent.

Introduction

We present a new, practical approach to well test analysis with the intent of improving the confidence in, and the accuracy of, reservoir properties generated from such tests.

Whether a geologist is analyzing a falloff well test from a permeable confined aquifer to estimate water supply or, an engineer is analyzing a production buildup test from a twenty-year old producing oil well, to identify any formation damage, the problems encountered during analysis are quite similar. Standard practice is to perform at least two types of analyses:

  • type-curve fitting on log-log or semi-log paper and,

  • semi-log straight line plots.

One tries to arrive at comparable values for transmissivity (hydrogeological term) r permeability (petroleum engineering term) from each method and normally report what is thought to be the best answer.

The method outlined below is intended to give a generalized approach to the coupling of H2O or more analytical techniques so that only one answer is feasible, while the requirements of curve matching or drawing of proper slopes are still met. The more unique the answer, the greater the confidence in using the values for formation and economic evaluations.

The technique used to essentially lock analysis methods together is described by example rather than as a stepwise procedure, the reason being that each set of analysis pairs, or groups, usually provide a slightly different or even uniqueapproach to obtaining the correct solution. For example, nalysis could concentrate on time intercepts rather than pressure match points or slopes.

Two examples are presented here, the first is well testing from a hydrogeologist's view of flow through an aquifer and the second is well testing from an engineer's perspective on water (or steam) injection into a highly oil saturated, very viscous oil sands reservoir.

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