Techbits: Refracturing in Low-Permeability Reservoirs

Abstract

A recent Applied Technology Workshop (ATW) held in San Antonio, Texas, "Refracturing in Low-Permeability Reservoirs," explored refracturing techniques and overall reservoir effects. The technical agenda was divided into eight segments: keynote, candidate recognition, rock mechanics, refracturing design, economics, case studies, refracturing unconventional reservoirs, and poster sessions. Kicking off the ATW was Chris Wright, President of Pinnacle Technologies, who brought participants up to date on where and how this domain surfaced, the various methods of candidate selection, and several brief case histories. While the technique of refracturing wells began in the 1940s, only a small number of wells have actually been refractured. The industry's tendency to refracture poor-quality wells has led to a perception that the technique of refracturing may not be successful. There may be as much as 10 Tcf of proved developed nonproducing gas reserves available by means of refracturing. The candidate selection process has improved significantly over the past few years, leading to better understanding and improved results. Various formations have responded to larger fractures, water fractures, low-gel-load fractures, and other methods. The data must be gathered and analyzed and best practices employed to optimize the refracturing design. Candidate Recognition Three presentations addressed selection of candidate reservoirs for refracturing. In a session titled "GIS Technology for Refrac Candidate Selection: Process and Example," BJ Services Applied Geoscience Manager Randy LaFollette described how a Geographical Information System (GIS) is used in the candidate selection process. The methodology is statistical, combining pattern recognition from aerial distribution of production performance and computational analyses/algorithms to provide short lists of refracture candidates. Individual well review occurs only on the "short list" wells as the last step before refracture treatments. A case history was used to explain workflow. West Virginia U. Professor Shahab Mohaghegh explained candidate selection methodology using virtual intelligence in a session titled "Restimulation Candidate Selection in Tight Formations." The process moves from field review to well-by-well review. This methodology attempts to merge a comprehensive production data set and analyze the data with reduced subjectivity while addressing the entire reservoir capability within reasonable geographic distance. The methodology is statistical and integrates several production-data-analysis techniques (e.g., type curve/dynamic contact angle). Case histories were discussed at the end of the presentation. In "A New Refracture Candidate Diagnostic Test Determines Reservoir Properties and Identifies Existing Conductive Damaged Fracture," Halliburton Rocky Mountain Tech Team member David Craig discussed a diagnostic test to determine refracturing candidates on both a well-by-well and an interval-by-interval basis. The test requires a short injection above fracture-initiation pressure and an extended shut-in period with the pressure falloff recorded. The test has both qualitative (identify existing fracture damage) and quantitative (permeability, current reservoir pressure, current fracture properties) objectives.