In Val Verde and Crockett counties of Texas, operators are drilling 15,000 feet plus wells to produce the Ellenburger formation. Bottomhole Static Temperatures are in the neighborhood of 320°F. Since the Ellenburger is a massive dolomite formation with natural fractures the optimum stimulation method is acid fracturing.
Discussed are the critical issues to achieve a successful stimulation using acid at plus 300°F. In addition, techniques employed will be reviewed. One technique is stimulation in a single treatment using ball sealers for diversion and CO2 for load recovery. A second method is to break up the large interval into smaller segments treating and isolating with plugs. The third technique employs packers and sleeves with CO2 for load recovery. The production results from these techniques are compared.
Stimulation of carbonate reservoirs is typically used to restore or enhance production to an economic level. Acid, whether organic or inorganic in composition is a natural means of effecting such stimulation of these types of lithologies.1–8 Acid fracturing is the most widely used technique for deep stimulation of moderate thick limestone or dolomite formations at moderate temperatures (<250°F). The use of acids at elevated temperatures with faster reaction times is a challenge.9 The additional complication of trying to cover a large interval (1000 - 1500 feet) makes the overall challenge of stimulation success extremely risky.
Core samples from a Val Verde county, Texas, Ellenburger well (~15,000 feet), representing 1200 feet of interval, were found to have solution collapse breccia, along with dolomitized limestone (Ankerite) and clearly defined bedding structures.10 Angular breccia fragments range in size from coarse-grained sand-like particles to large cobbles greater than eight inches in diameter. Numerous samples exhibited vugs between the framework fragments. The presence of natural fractures indicates tectonic activity and, since some of the older natural fractures are cross-cut by newer fractures, at least two periods of tectonic activity have occurred. These fractures varied in width from micro to over one half inch, the wider fractures having been enlarged by dissolution of bounding matrix rock. Many of the natural fractures are partially to completely filled with authigenic precipitates (calcite, dolomite and quartz). Some open micro fractures were incompletely filled by mineralization and probably represent the last tectonic episode. Fig. 1 breaks down the variance in lithological composition with depth.
The Ellenburger is a gas reservoir (0.69 to 0.73 gravities) with some water production. Gas composition is a mixture of methane and other light hydrocarbons plus high concentrations of CO2 and H2S. Original bottomhole pressure was found to vary from 3400 to 6400 psig. Bottomhole temperature in this area is over 300°F. The core samples referenced above had permeabilities varying from 0.1 md to over 10 md and porosity from 0.5 to 4.6% with averages of 2.39 md and 1.51% respectively.
Wells completed in the area are cased and perforated. In 1978 and 1979 wells were typically stimulated with 20,000 to 51,000 gallons of 20% hydrochloric acid and in two cases were fraced with 43,000 and 90,000 pounds of sand. Treatments were typically performed on two to three segments of the overall interval and tested for some period of time before proceeding to the next interval. These wells have produced from 1 to 10 BCF to date.
This paper presents details of the current treatment methods for stimulating newly drilled Ellenburger wells and the production responses observed.