Abstract Development of light-oil fields in Mexico is part of a strategic program to increase overall production in Mexico. Among the light-oil fields, operators have gas condensate carbonate reservoirs with medium permeabilities ranging from 3 to 10 md, with very high drawdowns. The initial development concern was how to make them produce above the bubble point. The first thought was to fracture the intervals to reduce the drawdown, but the permeability values from well tests made matrix stimulation appear acceptable. Some believe that with permeabilities in the 3 to 10 md range, fluid-loss issues may prevent reaching fracture pressure. In that case, a matrix treatment may be used, thus avoiding a more expensive hydraulic-fracture treatment.
However, step-rate test (SRT) results were unclear, so doubt remained. In three cases, permanent downhole sensors were available, and minifrac and fall-off analyses with downhole data were possible. Observation of the flow regimes in the data established a comparison of planned flow periods before and after the tests and frac jobs. If this data is used to help optimize treatment, applying frac treatments will be the best choice, and the investment will be justified. It was also observed that lower injection rates must be applied during the SRTs, starting from 0.5 bbl/min.
The operator concluded that "gray zone" candidate wells with medium permeabilities can benefit from matrix treatments and fracture treatments.
Post-fracture evaluation shows that drawdown matches very closely to the flow prediction, and this kind of reservoir can be produced above saturation points.
In this paper, three successful case histories are discussed in detail.
Introduction May Field is the principal gas-condensate producing field for the light-oil project Offshore Mexico. Oil and gas production is constrained by (1) the skin factor, (2) the amount of drawdown due to the low-permeability effect and (3) the limited presence of open fractures. With small acid treatments, wells could produce an average oil rate of 1,800 BOPD and 8 MMscf/D, with an average drawdown of 6,000 psi, as a result of skin elimination. However, saturation pressure became a concern, because producing a reservoir with high drawdown can create condensate in the reservoir, causing oil blockage.
The purpose of this work is to present an alternate use of real-time data with permanent sensors, applying minifrac analysis and well test techniques to help optimize the way we perform stimulation treatments in medium-permeability gas condensate reservoirs, i.e. "grey-zone stimulation candidates."
Reservoir Description May Field is a gas-condensate reservoir located 30 miles north from Dos Bocas, Tabasco, Mexico. It is part of a strategic program for light-oil production. May Field produces from Cretaceous and Jurassic formations with average thickness of 380 m. The limestone is compact and very clean with small sealed fractures and fissures, and is at a depth of 5,500 m. The average permeability is 3–8 md. The initial reservoir pressure was 11,715 psi, with a reservoir temperature of 176°C. The dew-point pressure is 5,623 psi, and the initial condensate gas ratio (CGR) was approximately 205 bbl/MMscf, and the gas oil ratio (GOR) was 890 m3/m3.
Early wells showed almost 60% of energy losses across the formation; initially it was blamed to skin. Skin factor with these permeabilities can be moderated between 20 and 50 (dimensionless).
Problem Approach After several attempts to enhance production using matrix treatments, the first questions were:How to optimize and enhance treatments?
Should fluids be changed?
Would rate increases be beneficial to the treatment?
Can the formation be broken with such high rate admissions?