Interpretation of Directional Gamma Ray Logging Data for Hydraulic Fracture Orientation

Abstract

Abstract A new directional gamma ray tool has been developed and is being incorporated into an innovative logging service. The service determines the azimuthal pattern of gamma ray emission from a completion material that has been tagged with a radioactive isotope. This paper discusses the interpretation of directional gamma ray logging data following hydraulic fracturing operations. By measuring the intensity of gamma rays emitted from tagged material placed in the induced fracture, the directional gamma ray measurement can be combined with auxiliary wellbore survey information to assist in the determination of propagation azimuth. Such results have a variety of applications - for example, designing a systematic well placement scheme to optimize reservoir drainage efficiency. Log examples derived from prototype tool measurements following hydraulic fracturing operations are presented and the analysis of the data is discussed. The interpretation of directional gamma ray data is adversely affected by damage to the near-wellbore region during fracturing operations. Data interpretation was found to be more consistent and reliable following micro-frac or mini-frac stress determination operations. In addition, where perforations were oriented in the direction of the anticipated fracture propagation, directional gamma ray data demonstrated an improvement in fracture efficiency. Introduction The theory of rock mechanics has been used to help predict the geometric parameters of fracture length, width and height. It has been established that the direction of fracture propagation is parallel to the maximum horizontal stress. The knowledge of the azimuthal direction of the hydraulic fracture propagation coupled with fracture modeling predictions can be used to design a well spacing and alignment pattern that would optimize the drainage of a reservoir. However, fracture propagation azimuth has been difficult and expensive to obtain and is not routinely used in development programs. Techniques for determining the orientation of hydraulic fractures include:An elastic strain relaxation measurements from oriented whole cores,characterization of borehole breakout features,borehole imaging using acoustic, electromagnetic and optical devices,borehole extensometer measurements made during micro-frac operations,passive and active seismic monitoring in both borehole and surface environments,measurements of surface displacements with tiltmeters andsurveys of magnetic anomalies with magnetometers. It has long been thought that the direction of hydraulic fracture orientation could be determined by making azimuthal gamma ray measurements when radioactive tracer materials were placed in hydraulically induced fractures. More than a decade ago oriented gamma ray tools, which were originally used to orient perforating guns (used for perforating in tubingless completions), were adapted to make directional gamma ray measurements for fracture azimuth orientation. These early tools, including one used by HLS, used rather low efficiency Geiger-Mueller detectors. Statistical variations and low count rates from these detectors gave poor results. P. 95^