A Diagnostic Technique For Restimulation Candidate Selection
Authors: Crowell Ronald F. , Jennings Alfred R. 
Published: December 2007

This paper presents a method to determine the present condition of a refracturing candidate gas present condition of a refracturing candidate gas well and allows the operator to decide if further stimulation is warranted. This method involves the use of type curves, computer analyses of the original treatment, and a logical comparison of the results. By using constant pressure and constant rate-type curves to analyze actual well performance and build-up data, the effective performance and build-up data, the effective fracture lengths and fracture conductivities created by the original fracturing treatments were calculated. Computer analyses of these original treatments were used to estimate created fracture lengths and, in conjunction with laboratory data, fracture flow capacities. A comparison of these fracture parameters allowed a better understanding of the current performance and resulted in successful refracturing treatments. Lack of agreement between type curves and computer analyses would have indicated potential problem areas such as proppant transport or lack of fracture confinement. proppant transport or lack of fracture confinement

Introduction

Restimulation efforts in low permeability gas wells generally have met with only limited success. Frequently, restimulation is based solely on the fact that unsatisfactory performance has been obtained and the most logical solution is to retreat the well, often with the operator realizing the marginal chance for success. To make an evaluation of restimulation potential, it is necessary to evaluate current performance in light of the original fracture treatment. Knowledge of fracture length and fracture conductivity, in conjunction with formation permeability, reservoir pressure and productive sand characteristics, pressure and productive sand characteristics, should allow assessment of the original stimulation and indicate potential for refracturing success. Increased fracture length or higher fracture conductivity may result in better performance from the candidate well.

RESTIMULATION CRITERIA AND GUIDELINES

Determination of the present condition of a refrac candidate should be by two independent techniques arriving at a common indication of fracture length and fracture conductivity. One technique involves the analysis of performance data to determine the "effective" fracture length and conductivity. The other technique involves the use of fracture stimulation design concepts including computer programs to assess the "created" fracture length and conductivity. Agreement between the "effective" fracture length and the "Created" fracture length implies that certain fracture characteristics are known. Lack of agreement implies that potential problem areas exist and that further consideration is necessary prior to restimulation. prior to restimulation. Performance analysis: When considering a refrac treatment for a specific well, one of the first considerations should be to determine if the well responded to the original treatment. This would be dependent primarily upon the resource potential of the reservoir. Restimulation design potential of the reservoir. Restimulation design should always consider the possibility that the well lacks sufficient reservoir energy or economically recoverable reserves to be a viable refrac candidate. If the well responded to an original stimulation, the chance for successful refracturing is enhanced.

Build-up tests had been used for a number of years in restimulation analysis by calculating a damaged condition or a created fracture length, depending on the technique employed. Performance analysis by pressure transient methods using "infinite" conductivity fractures often resulted in shorter fracture lengths than could possibly have been created by the hydraulic fracturing stimulations.

Proper evalation of restimulation potential should include the concept of finite capacity fractures.