Trawick field has been rejuvenated through the implementation of new completion strategies and an improved understanding of the reservoirs. Trawick is located in Eastern Texas and is a mature gas field with initial development beginning in 1949. The primary productive intervals are the Travis Peak, Pettit, and James Lime. The Travis Peak and James are both considered to be "tight" gas reservoirs with permeability averaging less than 0.1md in most wells.
By late 2001, cumulative gas production was nearly 1.2 Tscfg. Daily production had fallen to less than 40 MMscfg and was declining at a rate of 12% per year. The Travis Peak and James accounted for more than 75% of daily production at that time. The Pettit was thought to be mostly depleted, so additional production buildup potential was from the James and Travis Peak.
A redevelopment program was initiated in late 2001 to test the feasibility of horizontal completions in the James reservoir. The first few wells performed below expectations. Several mechanical problems were also encountered during drilling and completion operations. An integrated technical effort was initiated to improve the program performance. A high quality 3D seismic survey was acquired and analytic reservoir models were constructed to simulate existing wells. The models and seismic combined to enhance understanding of the reservoir and point out improvements regarding wellbore design, location, and completion techniques that would increase performance of future wells.
The resulting 31 wells (30 horizontal and 1 vertical) were completed in the James by August of 2005 with total daily gas production exceeding 60 MMscfg/D. The drilling program is ongoing and the Travis Peak is now under redevelopment.
Trawick gas field is positioned on a gently dipping, salt-cored dome. Production has occurred from six intervals ranging in age from Upper Jurassic to Lower Cretaceous. Major horizons are the Travis Peak, Pettit and James Lime. Minor productive intervals are the Haynesville, Cotton Valley, and Rodessa. The stratigraphic column1 (Figure 1) illustrates the relative position and age of each. The Cotton Valley and Travis Peak are sandstone formations, and the remaining intervals are carbonates.
Initial production was established from the Pettit in January, 1949. The Pettit is the dominant reservoir in terms of cumulative production, having produced about 640 Bscfg. This is driven by the relatively high reservoir quality (60–100 feet net pay, 15%-18% porosity, up to 200md permeability). Production from the deeper Travis Peak began in the early 1960's. The Travis Peak is noticeably lower in quality, with average porosity of 8% and average permeability less than 0.1md; however the net reservoir is more than twice as thick as the Pettit. Cumulative production to-date from the Travis Peak is about 410 Bscfg. Minor production was also established from the Rodessa, Haynesville, and Cotton Valley during this time frame.
Although the James Lime is shallower than both the Pettit and the Travis Peak, its development did not commence until 1977, 28 years after being penetrated by the field discovery well. Only after several wells in the Pettit and the Travis Peak had declined significantly did the James, with its poorer reservoir quality, become competitive to produce. The James has about 50 feet of net reservoir on average and possesses porosity and permeability values similar to the Travis Peak. Cumulative production to-date is about 150 Bscfg.
In 1997 the Trawick field was thought to be fully developed. By that time the three primary reservoirs had been drilled to either 160 or 320 acre spacing, depending on the structural location, and a field wide compression system was in place which allowed wells to produce to the gathering system at a pressure as low as 40 psi. Minimal new development occurred after 1997 and, as a result, total field production had fallen to less than 40 MMscfg/D by year end 2001 and was continuing to decline at 12% annually (Figure 2). End of economic field life was anticipated within 10 years, unless additional production buildup could be achieved.