Since its first use about 15 years ago, drill cuttings injection has grown to become a routine operation performed around the world. This rapid development is due to increasingly stringent environmental regulations and improved costs compared to alternative disposal options. The regulations for overboard discharge of cuttings started with limitations on oil-based muds and has now expanded to include synthetic muds as well. The economics of injection operations have improved as the industry has gained experience with the technology, with added benefits from the economies of scale offered by large capacity injection facilities.
The two critical engineering questions that must be answered for injection applications are:
where does the waste go and
how much can we safely inject into a well? These questions require integration of a variety of environmental, technical and economic factors that are important for each particular injection operation.The geometry of hydraulic fractures created during solids injection is one of the key issues in most operations. This coupling between solids injection and fracturing is the focus of this paper.
Cuttings injection began in the mid-1980's with small volume annulus injection in the Gulf of Mexico.1,2 By the early 1990's, it had already gained broader use in the GOM3, the North Sea4, Alaska5, and for NORM (Naturally Occurring Radioactive Material) disposal6. In the mid-1990's, the first large commercial facility with dedicated injection wells began operation7,8. This was followed by large-scale injection operations in Alaska9 and the Gulf of Mexico10,11,12. At present, annulus injection is available for routine use offshore, with several different service companies providing a range of operations and engineering support. An example of the continued evolution of the technology was documented in a 2002 study on commingled drill cuttings and produced water injection13.
The two most common sources of waste injected are from ongoing drilling operations and from mud and cuttings that have been temporarily stockpiled pending some future permanent disposition. Cuttings from an ongoing drilling operation are usually retrieved from the shale shaker, mixed with water, processed to an appropriate size and injected downhole. In the US Gulf of Mexico, for example, over 1000 wells were drilled in 1998. Each one of these wells generated cuttings commensurate with the hole-volume drilled, usually at least 1000 barrels of solid cuttings, or about 3000 barrels of slurry. By comparison, there are only a few locations where cuttings have been stockpiled, but the volumes in these sites are enormous. On the North Slope of Alaska, cuttings from wells drilled in the 1970's and 80's have been stored in reserve pits at dozens of drill sites. By 1993, the stored volume had grown to about 5 million cubic yards of mud and cuttings, or about 15 billion pounds of solid cuttings.
The ability to inject solids depends primarily on the subsurface geology, with the chemical composition or physical properties of the solids a secondary factor. For drill cuttings injection, the solids will consist of rock from whatever geologic section has been drilled: this could be sandstone, shale, limestone, coal, dolomite, etc. Cuttings will always contain some residual mud carry-over, including the base mud, plus any fluid loss additives, polymers, barite, etc.All these can be readily processed and injected. In addition, NORM solids have been injected in both small annulus injection batches and large volume dedicated injection wells. If regulations allowed it, both non-hazardous and hazardous solid waste can also be injected.