Advances in Practical Well-Test Analysis (includes associated paper 26134 )
Authors: Ramey H.J. 
Published: December 2007

Distinguished Author Series articles are general, descriptiverepresentations that summarize the state of the art in an area of technology bydescribing recent developments for readers who are not specialists in thetopics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and presentspecific details only to illustrate the technology. Purpose: to informthe general readership of recent advances in various areas of petroleumengineering.

The decade of the eighties produced important findings in well testanalysis. However, the large number of new and sometimes competitive methodsalso has produced confusion. The main objective of this paper is to considerthe current state of practical well test analysis methods. Often new studiesproduce conclusions that time proves incomplete or partly untrue. The storagelog type curve was initially presented as a method to analyze shorttime data. This was later found to be impossible. But the diagnose tic value of thelog-log curves was far more important than shorttime analysis. Then later thederivative was added to the type curve and one conclusion was that Hornercurved were no longer necessary and that short time analysis was now possible. Neither conclusion is entirely correct, yet the diagnostic value of thederivative remains. Another major development was computer aidedinterpretation. The computer was necessary to differentiate data, and of greathelp in preparing the large number of graph required for modern interpretation. An important breakthrough resulted with development of nonlinear regression forspecific models and the ability to consider rate variation. Results of aninterpretation could be used to simulate the test data and a comparison ofsimulation and field data made. The regression coefficient or confidence limitprovided a quantitative measure of the agreement of field data with the modelchosen. Results may also be used to determine where a correct straightlinecould be on a Homer buildup graph. This procedure proven that it is verydifficult to find a Horner straightline slope with the precision previuslythought possible. Widespread use of electronic pressure gauges and computerdata acquisition has created net problems for a well test analyst. A newproblem revealed by type curve analysis is presented.

Introduction

A 1976 study reviewed important findings of the previous decade and stressedpractical applications of new methods not described in the originalpublications. At that time. three log-log versions of the storage and skinproblem had appeared, and several type curves for fractured wells had beenpresented. Many well test analysts threw up their hands at the proliferation oftype curves. The 1976 study concluded that a major advantage of the log-logtype curve was that it was usually possible to identify the flow model and findthe start of a semilog straightline for the appropriate model. It wasrecommended that a Horner straightline should still be used as the final basisof analysis where possible. The 1976 study pointed out problems with fracturetype curves (short apparent fracture lengths for large jobs), and otherexisting worries on selection of an appropriate type curve. One major advantageof the understanding that new methods brought was that it was possible tocorrect bad test data and fill in missing data in many cases. Since 1976, problems identified in that study have been solved. and true breakthroughspresented. The purpose of this study is to present useful practical methods forwell test analysis and design. Important new information includes selection ofan industry standard storage type curve. development of derivative methods, solution of the finite fracture conductivity problem. and development ofcomputer aided interpretation and design.

STORAGE TYPE CURVE

The traditional engineering dilemma is two methods to solve a problem whichyield different answers. Well test analysis is replete with this problem. Papers often say something like the data was analyzed by the Smith methodyielding 20 md, the Jones method yielding 2 darcies, and the von Schultz typecurve yielding 0.15 md. The average is …" Remarkably, the three differentmethods are usually different graphs of the same solution. The second well testanalysis monograph by R. C. Earlougher, Jr. was published in 1977, but thelog-log type curves described for the first time in an SPE monograph wereconsidered controversial by the SPE Board of Directors. An early SPE Boarddecision was not to publish full-scale log-log type curves as this wouldindicate SPE approval of log-log type curves, or a particular type curve. Fortunately, this decision was reversed. Gringarten et al. ended thecontroversy over the best form of the wellbore storage and skin effect typecurve in 1978. Their type curve of the log of dimensionless pressure vs the logof tD/CD with CD exp 2s as a parameter has become the industry standard. Theiroriginal type curve combined radial flow and fracture flow results andindicated the effect of producing time on buildings. It was a remarkableimprovement which caught immediate acceptance. So one problem identified inref. 1 was solved.