Establishing Target Oil in a Major Oil Field Under Gas Assisted Gravity Drainage

Abstract

Abstract Technical analysis and data gathering methods are presented for establishing the lowest depth of development in a shallow, highly fractured San Andres dolomite field currently being developed under Gas Assisted Gravity Drainage (GAGD). Expanding development deeper is critical for extending the field life and managing long term drilling and CO2 injection. The field underwent a first phase of GAGD from 1992 through 2000 restoring the oil-water contact down to the discovery depth of +1050 feet above sea level (ASL). Several steps were taken to quantify the transition zone target for expansion including field wide data gathering, areal performance analysis, and a dedicated pilot or demonstration project. Data gathering included saturation logging of several existing open-hole observation wells. Pilot hole drilling and logging prior to horizontal well re-drills was also utilized. Dimensionless performance analysis and modeling were used to compare both areal and vertical variations in development. Finally, an extensive pilot project was initiated in 2021 which included a de-watering test, sponge coring and & laboratory analysis, horizontal drilling and repeat saturation logging to demonstrate the feasibility and commerciality of deeper development. Logging showed oil saturations as high as 50% down to a range of depths from +1000 to +990 feet ASL and rapid declining saturation to the imbibition free water level of +960 ASL. Deeper rock properties also showed similar reservoir porosity, permeability and fracture intensity as the shallower horizons. Performance analysis showed varied oil response closely tied to fracture intensity and oil column thickness (OCT). The demonstration pilot had several key findings: 1) de-watering of the reservoir to expand the GAGD process and gas cap could be established within several months, 2) the lowest depth of mobile (or target) oil is +990 feet ASL, 3) horizontal wells drilled to a depth of +990 feet ASL showed two distinct oil responses: a) oil production from reconnecting the oil column plus additional drained oil from contact lowering and b) 100% water production for several months prior to oiling in from contact lowering. The two distinct responses also coincided with OCT in the fractures and oil-water contact level prior to deepening. The combined response was observed in the Eastern part of the pilot with 20 foot OCT and OWC at +985 feet ASL versus the Western part of the pilot at 5 foot OCT and OWC varying from +1005 to +990 feet ASL. Prior to field testing development deeper into the transition zone, GAGD core laboratory experiments showed remaining oil saturation as low as 15% could be achieved. The intensive data gathering and field demonstration showed incremental reserves could be achieved. This has resulted in extending the drilling program several more years, lowering of the contacts further and additional oil recovery.