A Practical Method For Improving the Accuracy of Well Test Analyses Through Analytical Convergence
Authors: Leshchyshyn T.H. , Ali S.M. Farouq , Chan M.Y.S. 
Published: March 2010

Well test analyses are normally performed using a combination of type-curve and semi-log methods for determining reservoir rock properties such as permeability (transmissivity) or near-wellbore damage. Unfortunately, calculated values for these properties can vary substantially between the two methods. Conducting a history match can often eliminate ambiguity but it fails to identify the best answer from multiple solutions.

A practical approach for improving reservoir characterization is introduced in this paper which allows a unique solution to be found for two or more well test analysis methods. The technique couples the multiple solution parameters to derive a testing criterion for the unique solution, i.e., two equations are combined into one with the single formulation being used to determine whether the analysed data points meet certain requirements simultaneously between the two or more solutions. It is assumed that initial boundary conditions used in deriving the separate, method-dependent solutions are identical or very similar.

Two field situations are presented, one from a high transmissivity groundwater flow (water source) and the other from a low permeability oil sands reservoir. The cases, which are representative of a wide range of natural reservoir systems, serve to demonstrate both the universality of the new integrated method and the actual change from singular results.

For example, values for transmissivity using two different analysis methods varied by 20%, but reanalysing the data with the composite technique reduced the difference to less than 5%. Also, a previously reported value of formation compressibility was changed by 100% after determining a more accurate value for permeability.

Introduction

A new, practical approach to well test analysis is presented for improving the confidence in, and the accuracy of, reservoir properties generated from such tests.

Whether a geologist is investigating a fall-off well test from a permeable, confined aquifer to estimate water supply, or an engineer is studying a production buildup test from a 20-year old producing oil well to identify any formation damage, the problems encountered during analysis are very similar. Standard practice involves performing at least two types of analyses:type-curve fitting on log-log or semi-log paper and,semi-log straight line plots.

A comparison of the values for transmissivity (hydrogeological term) or permeability (petroleum engineering term) from each analysis is normally done to reduce the estimates to what is thought to be the best answer.

The process outlined below is intended to give a generalized method for coupling two or more analytical techniques so that only one answer is feasible, while the requirements of drawing proper slopes or curve matching 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 step-wise procedure, the reason being that each set of analysis pairs, or groups, usually provides a slightly different or even unique way to obtain the correct solution. For instance, analyses could concentrate on time intercepts rather than pressure match points or slopes.