By Steve Coombs, Pacific Operators Offshore, Inc., Santa Barbara, California

Figure 1 The offshore platforms at Carpinteria

Phillips rapidly developed the lease, bringing production to its peak of 27,900 barrels per day in mid-1969. Thereafter, production began to decline until, in 1990, with production reduced to an average 1440 barrels per day, Phillips decided to divest the property. Seeing great potential for improved performance, both from mechanical efficiencies and from additional drilling, Signal Hill Service, Inc. purchased the lease and its platforms and established Pacific Operators Offshore, Inc, (POOI) as operator. POOI immediately began an aggressive program of workovers, which improved production significantly. Simultaneously, POOI began studying ways by which big production gains could be made through a judicious program of drilling.

Figure 2 Location map of Carpinteria field showing Hogan, Houchin, Henry, Hope, and Heidi platforms

The field was on an anticline whose axis dipped about six degrees to the east. The producing rocks were turbidite sandstones and related deposits that were grouped into 28 discrete producing zones, some nearly 200 ft thick. Prior drilling had created an average well spacing of about 200 feet and nearly all were multiple zone completions with commingled production. POOI figured that the then relatively new technology of horizontal drilling might offer an opportunity to increase production by tapping bypassed oil in individual zones along the edges of the existing wells' drainage radii. This would also increase production efficiency by avoiding the high drawdown of conventional vertical completions, spreading production over a long, single zone horizontal perforated interval.

In 1994 POOI developed a horizontal drilling strategy and submitted a proposal to the Department of Energy under their Class III cost sharing program entitled "Feasibility of Optimizing Recovery and Reserves from a Mature and Geological Complex Multiple Turbidite Offshore California Reservoir Through the Drilling and Completion of a Trilateral Horizontal Well". The proposal was accepted and POOI formed a study team to prepare for the drilling of the well.

At the same time, POOI entered into a cooperative research program, also under DOE, called the Advanced Reservoir Management program, to develop a detailed geologic and reservoir characterization of the Carpinteria field in cooperation with Los Alamos National Laboratory, United States Minerals Management Service, California State Lands Commission, and the University of Southern California. The ARM study resulted in a complete petrophysical interpretation of all well logs in the field, a re-correlation and structural mapping of all zone tops, a new field-wide fault interpretation, and a 3-dimensional computer model integrating the structural data with porosity and oil saturation distributions, faulting, and well path geometry (Figure 2). These field-wide studies provided a strong foundation for the more detailed, lease-specific work performed by the study team on P 0166 during Class III Budget Period I. Careful re-mapping of the lease, particularly study of faulting patterns in relation to saturation indicators, showed that the project's objectives would not be best served by a single tri-lateral well, but rather by a series of individual redrills from existing boreholes.

At the completion of these studies in 1997 POOI entered Budget Period II of the Class III contract. Based on Budget Period I work results, POOI requested and received permission to modify the project to consist of the redrilling of a number of existing vertical producers into a horizontal configuration, creating drilling economies by allowing the use of thousands of feet of existing casing and providing a more thorough test of the horizontal drilling concept by penetrating a variety of zones with varying reservoir properties. POOI began by porting the 3-D model into their own modeling software, modifying the model to concentrate on the lease P 0166 area where the drilling was to occur. The model would be used to study the details of saturation and porosity distribution, to lay out horizontal redrill paths to evaluate the best zones while avoiding collisions with the many exiting vertical wells, and to demonstrate concepts and plans to project participants. The well data which had been used to create the maps and models represented a "snapshot" of the condition of the field at the time those wells were drilled, after which much production had occurred. Little was known with certainty about the position of today's oil/water contacts. To gain as much information as possible before drilling, POOI examined all the logs in all the zones on the lease looking for oil/water contacts. This work showed that the oil/water contacts in most if not all of the zones were tilted downward in the same direction as the dip of the structure, complicating the picture tremendously. POOI then mapped all of the saturation indicators in two dimensions for each zone to show where the oil had been when the field was first drilled and to see if any of the more recent wells would indicate depletion or water encroachment. The results were helpful but not conclusive. It became apparent that horizontal redrills could not be safely targeted until vertical pilot wells had been drilled into each fault block to demonstrate that good oil saturation still remained in the target zones. Fortunately the pilot wells could be completed in whatever zones were found to be saturated and would serve as conventional infill wells.

Mechanical considerations dictated that drilling begin from Platform Hogan, and POOI concentrated its detailed re-mapping efforts in that area. Two fault blocks were selected and, using the 3-D model and the detailed saturation and structural maps, horizontal well plans were laid out in the most likely zones (Figure 3). The planning of a series of likely horizontals even before drilling the pilot wells was necessary to prevent making the rig stand by while the results of the pilots were studied. It was a pretty sure bet that at least some of the proposed redrills would be proved up by the pilots, and drilling could begin on them immediately. The first pilot well was begun in June 1998. A second pilot into a separate fault block followed immediately. These wells showed the wisdom of the pilot well approach since one of the target zones was found to be very weak. The pilots did verify several of the other preplanned targets, however, and also showed strong saturation in a zone that had not been considered a good candidate from previous work.

Figure 2 A graphic image of the six horizontal redrills undertaken in the DOE-funded program.

With the pilot wells completed, drilling began on the horizontals. Part of the pre-planning process had been to find wells with poor or no current production whose well bores pointed in the right direction for redrilling to one or more of the target areas. Those wells had been selected and windows had been milled in the 7-inch casings to permit the directional drilling of the horizontals. The horizontals were drilled with slimhole tools, utilizing measurement while drilling techniques. One of the challenges of drilling these wells was that rather than being actually horizontal, they were planned to take a curving path through the zone in order to intersect most or all of the many sub-units of which the zones are made. In many cases, the inclination angles of the redrills were negatidepth. The wells were logged while drilling with both gamma ray and resistivity data being recorded. Since the wells could be evaluated as they were drilled there was no delay between the end of the drilling phase and the beginning of completion, which was done utilizing slotted liner.

In all, four redrills were completed in two fault blocks and three different zones. Including production from the conventionally complete pilot wells, these redrills added close to 750 barrels per day, nearly doubling Platform Hogan's production. These results exceeded POOI's pre-drilling estimates and verified the concept of improved production in mature turbidite reservoirs by horizontal drilling. The detailed geologic picture developed under Budget Period I of the Class III project indicates the potential for as many as thirty more targets for similar re-development on lease P 0166. POOI is currently developing plans to pursue these opportunities.

The Class Act Summer 2000 Edition Volume 6/2