Pulsed-neutron capture logs (PNC) have been applied to address specific reservoir management issues in three mature Permian Basin fields (Cordona Lake, Cornell, and East Vealmoor). When combined with existing well data and a knowledge of formation fluid compositions, single runs with PNC logs have provided valuable reservoir surveillance data in secondary and tertiary recovery reservoirs. The logs have led directly to increased production, more cost effective workovers, and an improved understanding of fluid contact movement and injection breakthrough.
Pulsed-neutron capture log technology permitting fluid typing in cased hole has been around for over two decades1,2. However, recent improvements in compensating for borehole signal and accounting for diffusion effects3 have expanded the conditions in which credible measurements can be obtained4,5. In the past, fluid saturation modeling with PNC tools in a non-monitor environment were generally restricted to conditions where the product [(Swater-Soil)*PHI>3.0]5. The newer tools are often capable of identifying fluids when this product is as low as 2.0. This improvement has had a dramatic effect on PNC use in the Permian Basin where low porosity and/or low or unknown formation water salinity had limited past PNC application.
Ideally, long term reservoir surveillance programs involving regularly scheduled injection profile monitoring and repeated logging of observation wells provide the best opportunity for monitoring fluid contacts and CO2 movement in a dynamic reservoir6. However, such programs are often judged too costly, or are disrupted by cost control programs. In these instances, judicious use of single PNC logging runs, in conjunction with existing well and field data can provide the necessary data. This paper details the results of three such efforts in maturing Permian Basin reservoirs.
The Cordona Lake Unit exploits 150'-250' of Devonian tripolitic chert sitting unconformably below the Permian Wichita-Albany dolomites. The Crane County field was discovered in 1949 and waterflooded in 1967. A tertiary WAG recovery program was initiated in 1985. Currently, the high porosity (20-25%), low permeability (5-9 md) chert is being exploited in 10-20 acre line drive patterns.
To maximize production, the original wells were positioned at highs in the unconformity surface where the prospective Devonian section is thickest. As a result, many of the injectors are located in lows which permit support to only the middle and lower intervals. Within the field area, paleo-topographic relief on the unconformity surface is as much as 250 feet.
In February, 1994, a TDT-P log was run in a flowing producer to monitor water and CO2 breakthrough and to evaluate unperforated interval. The 1982 well had core and open-hole log data with which to compare the PNC response. The well had been originally completed to 5500, but yielded only water and was shut-in until 1989. At that time, the current perforations (5318-5440) were opened and the well became a producer. Production was erratic until a restimulation in 1993 led to sustained production. The success of the 1993 restimulation is thought to reflect the advancement of the tertiary front to the location. At the time of logging, this well was producing 660 BOPD, 240 BWPD, and 130 MCFPD; total production was 47 MBO.