Mechanisms of Shallow Waterflows and Drilling Practices for Intervention

Abstract Geopressured water sands near the mudline in deepwater, greater than 1000 feet, have been shown to be hazards when these sands are permitted to flow outside structural and 20" conductor pipe. Special drilling practices are required to contain the pressure during drilling and casing operations. Four mechanisms have been identified as causes of shallow waterflows:induced fractures,induced storage,geopressured sands andtransmission of geopressure through cement channels. Geoscience techniques have been developed to aid in the detection of the shallow waterflow mechanisms, prior to drilling. These techniques include seismic stratigraphic interpretation of shallow hazard airgun data and specially processed 3-D surveys, and special pore pressure and fracture gradient prediction methods. Drilling and cementing practices have been developed to minimize the risk of inducing flow behind structural and conductor casings. Each of these flow mechanisms require adapted drilling and cementing practices to prevent potentially damaging flow. This paper presents best practices developed by this operator along with our contractors to detect, drill, case, and cement shallow waterflows in the deepwater Gulf of Mexico. These practices should be transferable to other similar sites around the world. Introduction A shallow waterflow can be defined as water flowing on the outside of structural casing to the ocean floor. This flowing water can erode the structural support of the well which may lead to buckling of the casing and subsequent casing failure. This flow path will also compromise the wellbore integritywhich can result in loss of well control. A study has been conducted on seventy-four (74) industry wells in areas of the deepwater Gulf of Mexico (GoM) wheregeopressured sands within 2000 feet of the mudline were known to exist. Well results associated with shallow waterflowincidents were classified into one of four categories:the well had a shallow waterflow which rendered the well unable to achieve its designed objectives.the well had a shallowwaterflow and was able LO reach planned total depth, but because of the flow, only limited formation evaluation was possible,the well had a shallow waterflow but successfully completed all of it's planned objectives, andthe well had noknown shallow waterflows. See table 1. Using this criteria, twelve percent (1 2%) were never able to reach the final objectives due to the occurrence of shallow waterflows. One percent (1%) of the wells were able to reach total depth, but evaluation was limited due to the influence of [he shallow waterflows. Fifty-three percent (53%) had known shallow watertfows but were able to complete all depth and evaluation objectives. Thirty-four percent (34%) of the wells did not encounter any shallow waterfows problems. In one of the more severe oases the formation fluid escaping through the shallow waterflow resulted in significant compaction of the shallow sand. The surface expression of this compaction and subsequent subsidence produced a fissure extending up to 300 feet from the wellhead with as much as seven feet of displacement between the two opposing sides of the trench.