Abstract

This paper presents an in-depth assessment of the fracture system created during the Mounds Drill Cuttings Injection Project. The project goal was to evaluate the adequacy of multiple injections in creating a "disposal domain". Previous papers have provided overviews of field results without supporting data or model calculations. This paper presents detailed core, log, tracer, and monitoring field data along with stress calculations, and shows that the Mounds disposal domain is more complicated and less-effective than would appear from the overview data.

The principal data defining the cuttings-filled fracture system are from cores drilled through the fracture system and imaging logs of fractures in the borehole walls. Supporting data include analyses of cuttings and induced fractures for tracers (radioactive, dye colorants, glass microspheres) injected with the slurry and fracture-diagnostic results from microseismic and tiltmeter arrays. While all field data show an extensive fracture system, core and log data show that only a few fractures have significant cuttings volume and fracture conductivity. The disposal domain created is one where there is a principal, narrow, cuttings-filled, fracture system, along with abundant, minimal-volume, offset, secondary fractures. Stress calculations also show that insufficient cuttings were injected to reorient the stress field and produce a wide range of fracture strikes. Secondary fractures probably formed from limited localized plugging or activation of intersected natural fractures. However, with sufficient additional injection, a wider disposal domain probably would have been created.

Introduction

The Mounds Drill Cuttings Injection Project was a joint-industry project organized by the Gas Research Institute (now Gas Technology Institute) and conducted in 1998 and 1999 at the Baker-Hughes Mounds Test Site near Mounds, Oklahoma. The overall project objective was to apply various advanced diagnostic technologies and verification techniques to identify the extent and characteristics of the disposal domain which was created during the batch injection of slurrified drill cuttings. An electronic media report compiling the available project data1 and several additional papers2–4 present overviews of the project background and interpreted results.

Upon initiating the project, there were alternate concepts regarding the potential character of the disposal domain. The disposal domain is defined as being those hydraulically induced fractures which accumulate the bulk volume of solids from multiple slurry injections.

One concept was that the disposal domain would be characterized by a complex network of slurry-filled fractures having a range of differing orientations. In this domain, the slurry would be deposited in a system of interconnected dendritic fractures with bifurcations which are propagated when horizontal stresses equalize due to the deposition of the slurry.

A second disposal-domain concept proposed that a limited number of induced fractures occurring in a relatively narrow strike range would accept the slurry from multiple injections.

Several papers5–6 have been published which lend support to the concept that the Mounds slurry-injection disposal domain may have occurred in a wide strike range. This appraisal of the results was based on overall diagnostic information from tiltmeters, microseismic data, and cored fractures that seem to support such a concept. A comparison of the results for these three diagnostic techniques1,6,7 is shown in Figure 1 for the Wilcox injection interval and in Figure 2 for the Atoka injection interval. Shown in each Figure are plan-view projections of the individual microseisms, the locations where cored hydraulic fractures were found, and the tiltmeter projections (strike from surface tiltmeters; length from downhole tiltmeters). A cursory glance at such data would suggest that a relatively widespread disposal domain was created.

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