fax 01-972-952-9435. AbstractConoco drilled the Spa Prospect, Walker Ridge 285 #1
Conoco drilled the Spa Prospect, Walker Ridge 285 #1, in the Gulf of Mexico to a depth of 29,452' MD / 29,434' TVD. The Spa Prospect was a subsalt well encountering approximately 9,981' of salt. The Transocean Deepwater Pathfinder, a dynamically positioned drillship, was utilized to drill this well in 6,654' of water. Original planned total depth for the well was 31,600' MD / 31,000' TVD. This represented one of the deepest wells ever planned in the Gulf of Mexico. All geologic objectives were reached by 29,452' and drilling operations were terminated. This paper describes the challenges involved with planning the well, documents the execution, and concludes with lessons learned. The well planning included the following:Location selection criteria for avoiding shallow hazards while meeting geological objections,Pre-drill pore pressure and fracture gradient estimation,Hydraulics design and its relationship to drill string selection,Casing and wellhead program objectives,Landing string design,Lost circulation assessment,Mitigation of annular pressure in trapped annuli, andThe implementation of test rams to reduce BOP testing times. The execution section of the paper describes experiences encountered and the technologies utilized. Introduction The Deepwater Pathfinder spudded the Spa well on January 29, 2002, in 6,654' of water. Figure 1 presents the planned casing depths with planned contingency strings, and the actual casing depths for the original hole and the bypass hole. The original well plan was to drill to 31,600' MD / 31,000 TVD (all depths in this paper are MD unless otherwise stated). This well plan measured 10,121 on the Dodson Mechanical Risk Index, a Gulf of Mexico industry-benchmarking tool. The original wellbore was drilled to 27,504' when hole problems led to a decision to bypass. The bypass hole drilled a step out from 14,992' to 29,452' and achieved all of the geologic objectives of the well. The original hole required 146 days to complete and the Days versus Depth curve is presented in Figure 2. The bypass hole required 82 days to complete and the Days vs Depth curve is presented in Figure 3. The rig crews are to be commended for achieving zero Medical Treatment Cases and zero Lost Workday Cases during the 228 days and 325,000 man-hours. Well Planning — Location Selection Seafloor location selection was based on avoiding shallow hazards and the ability to set 22" conductor casing in salt prior to installing BOP and riser. Entering salt as soon as possible reduced the need for the contingency 18" and 16" casing strings. The industry has realized thick salt sections act as a casing string. Two different seafloor locations were evaluated. Location #1 had a relatively flat seafloor with negligible shallow hazards potential but had no salt. Location #2 was in a highly faulted seafloor graben with a 700' escarpment that had a shallow, thick salt section, Figure 4. Detailed shallow hazards analyses were performed at location #2 and minimal shallow gas and water flow potential were predicted. Location #2 was chosen due to the ability to set 22" casing into salt. Pore Pressure and Fracture Gradient One of the greatest uncertainties in planning the well was predicting pore pressure and fracture gradient below the thick salt body. Pre-drill pore pressure estimates above salt were derived by analyzing seismic velocities at or near the proposed well location. Pre-drill pore pressure estimates below salt were derived by correlating seismic velocities at the proposed location to seismic velocities in the nearby abyssal basin where salt did not exist. The pressure estimates were derived using stacking velocities from the abyssal basin projected up underneath the salt body. Unfortunately this technique was highly reliant on the quality of the seismic data, and could not fully take into account disruptions in sediments caused by the emplacement of salt bodies. These types of disruptions could cause reduced pore pressure and fracture pressure below salt.
A long, tight clearance liner was recently installed through a whipstock-milled window in a deepwater well. The length and weight of the liner, coupled with the tight clearances, pushed the limits of current technology. The liner was successfully run through a deep whipstock milled window and cemented at a depth below 24,000'. This paper describes the planning and execution involved in this critical well construction operation. Introduction Installing a long, tight clearance liner through a milled window is a critical operation that presents many technical and operational challenges. Recently in the U.S. Gulf of Mexico, ConocoPhillips Gulf Region Deepwater Exploration milled a window in 13–5/8" casing and subsequently ran 9551' of 11–3/4" liner through a 12–1/4" window to a depth of 24,382' MD. Planning, equipment selection and proper execution were the keys to success. The first critical success factors were to construct a window with adequate clearance and minimal dogleg severity and then to provide a quality borehole below the window. A one-trip whipstock was selected to construct the window and provide rat hole below the window for drilling ahead. Various bottom hole assemblies were run to directionally drill and open the borehole to an interval depth of 24,382'. Prior to running the 11–3/4" liner, a borehole imaging survey was run to confirm suitable hole geometry and adequate clearance in the open hole interval. Considerable effort was made to select the proper equipment to run and cement the 11–3/4" liner. Finite element analysis was applied to select casing connections that would provide maximum clearance while maintaining integrity to support high tensile loads combined with severe bending across the window. In addition, extra attention was given to selecting auxiliary casing equipment needed to ensure that the liner would be successfully run to bottom and cemented. This paper illustrates the level of planning and detail required to successfully install a long, tight clearance liner. A case history of the planning and execution demonstrates the effort needed to successfully implement such a challenging operation. Planning Milling the Window Method and Equipment Selection After reviewing other sidetracking options including milling and casing recovery, a cased hole sidetrack utilizing a whipstock was selected as the most cost effective option with the lowest operational risk. A technically advanced whipstock system capable of producing a clean, full-length, full gauge usable window followed by sufficient rat hole to accommodate a rotary steerable drilling assembly in one trip was desired. Two types of whipstock systems were considered for the cased hole sidetrack, 1) conventional and 2) extended gauge multi-ramp design. The conventional whipstock usually has a 1–1/2 to 3 degree single ramp and a cylindrical shaped mill head dressed with crushed carbide. Inherent problems with the conventional design are, 1) inconsistency in the shape and location of the window, 2) the mill progresses slowly at the center point of the casing, and 3) formation imposed mill limitations. The extended gauge multi-ramp whipstock system features a specially designed whipstock face with multiple ramps, each with its own taper, and a milling tool with a conical shaped mill that can be dressed to accommodate different formation properties. This type of whipstock provides additional footage to the vertical face of the whipstock. The additional footage lengthens the usable portion of the window and reduces the dogleg through the window. By lengthening the vertical face of the whipstock, the departure angle of the milling tool is not sacrificed.
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