TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThis paper shows a successful automated system to control both steam rate and pressure in the heavy oil diatomite formations in the Midway Sunset field. Diatomite reservoir permeability is usually less than 10 md with naturally occurring fractures that promote premature steam breakthrough. In addition, this location was extremely shallow with the chance of surface steam eruption. The new system optimizes steam injection while controlling rapid pressure rise which could cause serious problems such as steam channeling and eruption out of zone.
This paper shows a successful automated system to control both steam rate and pressure in the heavy oil diatomite formations in the Midway Sunset field.Diatomite reservoir permeability is usually less than 10 md with naturally occurring fractures that promote premature steam breakthrough.In addition, this location was extremely shallow with the chance of surface steam eruption. The new system optimizes steam injection while controlling rapid pressure rise which could cause serious problems such as steam channeling and eruption out of zone. Introduction Due to the unique reservoir characteristics of the diatomite formation in the Midway Sunset Field, successful thermal development of this formation requires special operational considerations. The diatomite formation is a very porous soft rock with reservoir porosity values ranging between 50 and 75 percent.Reservoir permeability in the area of interest averages 3 md with naturally occurring fractures that could provide conduits for premature steam breakthrough between wellbores.Heavy oil diatomite formations in the Midway Sunset and Cymric Fields are typically thermally developed utilizing cyclic steam injection and flowback of the producers rather than steamflooding.[1]From an operational standpoint, continuous steam injection is difficult to maintain as the pressures to sustain constant rate injection rise rapidly.If done manually, choke size changes would be required almost weekly by the operator. A diatomite formation steamflood pilot was planned for implementation in 2004 on a Berry Petroleum lease at the north end of the Midway Sunset Field.This followed the drilling and completion of the Severini 834 well in 2003 in the SW _ of Sec 3 T31S R22E.Well 834 confirmed the diatomite net pay thickness, oil saturations, and the orientation of the natural fractures. The fracture orientation was measured using tilt meters during the hydraulic fracturing and was used to establish the pilot layout. This well had hundreds of feet of net pay, with pay being defined as exceeding 35 percent oil saturation.The pay interval in the area was divided by a wet diatomite interval of about 100 feet with approximately two-thirds of the pay above this interval and one-third below.The lower pay interval in the 834 well was siltier and had slightly lower oil saturations.Both intervals were considered Opal A although the lower zone had more porcelanite. The top of the upper pay interval varies in the pilot wells from 685 to 840 feet dipping to the southwest. Planning steam injection to avoid the wet zone was deemed critical to the success of any future project.Dual slimhole injectors were considered the best method to insure segregation of the steam into the two intervals. Oil gravity varies slightly from well to well in the pilot with the average being about 16.6°API. The oil gravity is approximately 2° lighter in the upper interval than the lower interval based on the results of core and sidewall oil sampling.This difference explains the well to well differences seen in the relatively small aerial extent of the pilot.All the pilot producers are completed in both pay intervals with the production being commingled. One objective of the pilot was to develop a method for injecting high quality consistent steam into the reservoir at the lowest pressures possible.Such a system would need to minimize the possibility of early steam breakthrough to the producers.Surface breakthrough or eruptions are also a possibility in this area given the shallow steamflood depth, relatively high injection pressures, and the reservoir's lack of a strong continuous capping shale or barrier rock.
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