The injection of drilling generated waste into a selected subsurface formation has evolved into the most preferred waste disposal technology in terms of environmental compatibility and cost effectiveness, especially for remote and environmentally sensitive areas. The main principle of waste injection (WI) is the initiation of hydraulic fracture and the placement of solids within the created fractures through the high-pressure pumping of slurry batches. The fracture propagation is governed mainly by in-situ stresses. As injection progresses solids accumulation in the fracture leads to an increase of in-situ stresses within the injection zone causing a build-up of injection pressure. As allowable injection pressure is often limited by equipment, the undesirable or rapid injection pressure build-up could jeopardize the operational life of an injection well and limit its waste disposal capacity.
This paper introduces a new approach to a regular seawater injection by investigating the relationship between pressure behavior and seawater injection. Displacement of slurry from the tubing by seawater over flush is carried out routinely in WI operations worldwide. However, never before it was considered as a pressure maintenance tool. The authors describe the impact of continuous seawater injection on injection pressure through the lifetime of three waste injectors. The injection pressure behavior before, during and after continuous seawater injection was reviewed using downhole measured data. It was noticed that regular seawater over-displacements during the continuous time periods between slurry injections reduced injection pressure considerably. Consequently, a thorough evaluation was initiated to investigate the impact of extended seawater injection on in-situ stresses and its potential advantage in maintaining the injection pressure within lower limits.
Considering the novelty and value of study for expanding worldwide WI operations, this paper presents the new approach to seawater injection as an injection pressure and disposal capacity maintenance tool.
In the early 1990s, WI emerged as a new technology that could provide an environmentally safe and economically sound solution to the disposal of drill cuttings and associated wastes in environmentally sensitive and remote operations. It was identified as one of the few technologies able to provide a complete drilling waste disposal solution that eliminates the need to accumulate, store and haul cuttings to shore for treatment. First pioneered with small volume annulus injections in the Gulf of Mexico in the mid-1980's the technology has since gained broad use in the North Sea, Alaska and other areas where the environmental conditions, tight regulations and logistics made this a viable drilling waste management option.1, 2
WI has been implemented successfully in the Caspian Sea since 2006, despite a complex subsurface environment and tectonics that presented significant technical challenges for vigilant monitoring and pressure interpretation as injection progresses.3 Pressure follow-up was recognized as critical to ensuring the safe and long-term containment of the waste to be injected.