Friday, June 8, 2018

Case Study Conformance Control

The Wertz field was a model implementation of a CO2 tertiary flood, and ,as a result, field performance had been copiously documented. Not only were individual producers and injectors monitored daily, but flow rates of the three phases present - oil, water and Co2 were also measured. These measurements were made in special substations, one substation for every dozen wells or so, each with elaborate and automatic appratuses for sampling each well flow in or out and the flow's breakdown into three phases. 

The Wertz producing formation is a 470-ft thick aeolian sandstone at an average depth of 6200 ft, with 240 ft of net pay  having 10% porosity and 13 md permeability. The formation is believed to have some fractures and is oil wet. Sixty-five wells over 1600 acres are used for production and many more that that have been drilled for injection-alternating water and CO2 injection, commonly referred to as water-alternating-gas (WAG) injection. By mid 1991, the field fate literally hung in the balance. The field's total production had dropped precipitously to 4000 BOPD from 12,000 BOPD in 1988, a steeper than expected decline during tertiary flooding. 

After trying several other techniques to halt the decline, Amoco turned to conformance control, eventually completing 12 treatments using Marathon's polymer gel technology. Ten treatments were in injectors and two in producers. Some treatments were aimed at blocking matrix porosity and some aimed to place gel in reservoir fractures. We'll highlight one example of each, illustrating with injector treatments since these were the more successful. In some cases, the treatments  extended the life of a pattern by two years. Overall, Amoco estimates that for a total cost of $936,000, the treatments have yielded an increase in producible reserves of 735,000 barrels.

 A crucial preliminary step in all these treatments was candidate selection- the compilation and review of data to determine a well's suitability for treatment. Although any field information could be relevant, five data types were deemed particulary imporant. They were:
  • Pattern reserves: If the pattern reserve data indicated that secondary and tertiary flooding had pushed out most of the oil, there was no reason to try further production enhancement with conformance control.
  •  Historical fluid-injection conformance. If an injection well historically showed a poor injection profile, the corresponding pattern was obviously a candidate for conformance improvement. In the Wertz field, Amoco used radioactive tracer surveys to log injection profiles.
  •  Three phase offset production data. If producing wells in a pattern showed a cyclic water and Co2 production that correlated with cycles in the nearby injection well, then it was likely this communication was through an unusally high-permeability channel. The pattern therefore required conformance control.
  • Breakthrough time during the cyclic correlation -essentially the time for water or Co2 to travel between injector and neighboring producer. This helped estimate the size of treatments designed to fill the fracture space between the wells.
  • Well history information- specifally the history of all previous attempts to improve conformance in the well, and why they did or did not work. This information prevented unnecessary workover expense.

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