|Publisher||Society of Petroleum Engineers||Language||English|
|Content Type||Journal Paper|
|Title||Use of Potassium/Lime Drilling-Fluid System in Navarin Basin Drilling|
|Authors||Holt, C.A., Amoco Production Co.; Brett, J.F., Amoco Production Co.; Johnson, J.B., Amoco Production Co.; Walker, T.O., O'Brien-Goins-Simpson and Assocs. Inc.|
|Journal||SPE Drilling Engineering|
|Volume||Volume 2, Number 4||Pages||323-330|
Summary. This paper presents a case history of Amoco Production Co.'s use of potassium-lime mud (KLM) for drilling a series of wells in the Navarin basin. The remote location, logistical concerns, environmental regulations, and the high cost of the operation mandated that the project be carefully planned. Planning, however, was hampered because the nearest offset to any of the wells was more than 100 miles [160 km] away. This paper describes the planning, implementation, and results of the mud system used to drill the wells. It includes a matrix of tests used to define further the nature of KLM, presents the methods used to run the system while the wells were drilled, and describes the results of using the mud in the basin. Navarin basin experience suggested that KLM should be considered when clay inhibition is needed and moderate bottomhole temperatures (BHT's) are expected.
The Navarin Basin
In 1985, we successfully drilled and evaluated five wells on leases acquired in the Navarin basin of the Bering Sea offshore Alaska (see Fig. 1). Ice coverage and environmental regulations limited the drilling season in the Navarin basin to the ice-free months of June through December. This problem, coupled with the remoteness of the basin, did not lend itself to a normally planned and executed drilling operation. As described by Zaremba et al., unique systems approach was used to organize, to plan, to implement, and to analyze the project. The resources from Amoco's Tulsa-based Critical Drilling Facility were selected to assist in project management and engineering. A field-based fleet, which included a ware ship and a tanker modified to handle the drilling-related goods and to make cargo and bulk-material transfers at sea, supplied most of the drilling operations. All material necessary for the entire drilling season needed to be loaded on the ware ship by late April to facilitate a June 1, 1985, spud date. In the area of drilling-fluid systems, a mud type needed to be selected by March 1, 1985, so that each mud product could be verified for purity and tested (for heavy metals and other contaminants) to ensure that the resultant fluid would conform to EPA Region 10 discharge standards. System selection was complicated further because only one well had previously been drilled in the Navarin basin (an area the size of Louisiana). That well, the Continental Offshore Stratigraphic Test (COST) well, was drilled near the center of the basin with 100-mile [160-km] closure to the company's closest prospect. While fluid selection was proceeding, a detailed analysis of the solids-control systems on each rig was completed with the Engineering Simulator for Drilling in the Tulsa research facility. The results of the study showed that upgrading the existing shale shakers and matching the hydrocyclones to the proper centrifugal pump/motor combination could more than pay for the expense of the modifications in mud costs alone. These modifications were completed on each rig before spudding and are discussed by Zaremba et al. The drilling fluid system selected for use in the 1985 Navarin basin drilling was the KLM system.
Planning for the Navarin Basin
Mud Requirements. Because the nearest offset was up to 100 miles [160 km] from the proposed locations, there was a large degree of geologic uncertainty. Also, because of the large intangible costs associated with a drilling program in the Navarin basin, the drilling fluid had to maximize the probability of reaching total proposed depth in the predicted number of days. Time delays for circulating and conditioning, and hole problems attributable to the mud, would not only be expensive but could also compromise the chance of drilling additional wells. Prudence dictated that the proposed mud he chemically and rheologically stable, and environmentally and intrinsically safe. The mud used in the Navarin basin needed to promote wellbore stability and have stable properties and logistical versatility in a wide range of possible drilling environments, as well as meet necessary approvals. These requirements are further described below. Approvals. Aside from any engineering considerations, the mud system selected needed to meet three other standards: (1) it must meet or exceed Environmental Protection Agency (EPA) discharge standards for Region 10; (2) to facilitate log interpretation, total salinity could not exceed 3,000 ppm; and (3) the use of lignite was prohibited to facilitate palynological interpretation. These constraints immediately reduced the number of possible mud types. Wellbore Stability. Significant hole problems in the COST well were attributable to hole instability. Reaming and hole trouble were directly attributed to unstable shale sequences. Caliper logs (Fig. 2a), COST well data, and adsorption tests on COST well cores documented that unstable shales exist in the Navarin basin. Also, COST well experience led the mud vendor on the job to recommend the use of a 5% KCl system in future Navarin basin work to overcome operational problems with the troublesome shales. In addition to the drilling problems caused by unstable shales and enlarged wellbores, log analysts desired an in-gauge wellbore for best possible interpretation of electric logs. Mud-System Stability. The system selected for use in the Navarin basin had to meet several system stability criteria that were important because of the uncertainty about the drilling environment that would be encountered. The system needed to maintain acceptable properties throughout a range of possible circumstances. COST well mud recaps demonstrated a significant problem with fine solids. The chemical dispersion necessary to control both rheology and fluid loss in the COST well, as a result of solids loading, also contributed to wellbore instability. In the 1985 Navarin drilling program, the solids loading situation would also be aggravated by a limited freshwater supply. This supply limitation, along with the permitted discharge limits of 10,000 gal/D [38 m3/d] and 42,000 gal [160 m3] four times during the life of the well, meant that mud problems could not be solved by dumping and diluting. The system selected must inhibit the yielding of formation solids long enough for the solids-control equipment to remove it from the system.
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