Design Considerations in Drill Cuttings Re-Injection Through Downhole Fracturing

Abstract

Abstract Upstream environmental policies require the use of best available and economically sound technology to continually reduce discharge of mud and cuttings waste resulting from drilling operations. Downhole disposal of mud and cuttings waste through hydraulic fracturing provides a zero discharge solution and eliminates future cleanup liabilities when the loop is closed. This downhole disposal technology has shown success in both onshore and offshore drilling operations and is becoming a routine disposal option of complying with environmental legislation concerning discharges of drilling waste. It also offers favorable economics. Careful job planning, proper operation design, strict quality assurance and systematic monitoring and analysis are the key to the success of waste disposal operations. Important drill cuttings re-injection engineering design and assurance process are presented in this paper. These include job planning, data required for subsurface geology or formation description and logging analysis, criteria for disposal well and injection zone selection and requirement for slurry rheology quality control. Also presented in this paper are design guidelines for injection strategy selection and injection parameters such as injection rate, batch size, shut-in time required for fracture to close on cuttings, injection equipment requirements, and disposal capacity estimates. Issues related to risk management and commingling of drilling waste injection and produced water injection in the same well are also briefly touched. Introduction Although the discharge of contaminated drilling wastes is dealt with through regional agreements or through national legislations, the general waste management guidelines follow the hierarchy of minimization, reduction, recycle and recover, treatment, and finally disposal or discharge. Within this framework of available options, the most acceptable form of waste management is to actually minimize the amount of waste being produced. The next most favored option would be to convert it into a usable product, then to treat it to make it non-harmful. Disposal of waste is seen as the least preferable solution. It should justifiably be argued that drill cuttings re-injection (DCRI) is a means of waste reduction as no drilling waste is left at the end of operations. Furthermore, in a broader sense, the waste material has been returned to its place of origin. It is for this reason that this drilling waste disposal operation through downhole injection is often known as drill cuttings re-injection, although one can justifiably call it drill cuttings injection. Recent environmental initiatives have been announced by some of the major operating companies aimed at green operation/zero emission targets. Injection of oily cuttings and associated wastes into subsurface geological formation often eliminates transportation needs and therefore spill risks over environmental sensitive area. It can achieve zero emission or discharge. More importantly, one can eliminate future liabilities with this technology when the injection loop is closed. It also offers favorable economics.1,2,3 For these reasons, drill cutting re-injection is becoming a routine drilling waste disposal method for complying with environmental legislation or company policy. There have been many successful injection operations conducted by major oil companies like BP-Amoco-ARCO, Chevron, Conoco, ExxonMobil, Phillips, Shell, Statoil, TotalElfFina, to name a few. A number of papers on this technology have been published since late 1980s to address case studies, equipment, economic, regulation and permitting issues.1–14 Laboratory testings on large blocks (1 meter cube) were performed to understand important fracture containment mechanisms and to verify multiple fracture creation during periodic injections of drilling waste.14 Field trial of cuttings slurry injections and coring from near-by wells confirms creation of multiple fractures from periodic slurry injections into sandstone or shale formations.16–18