The Black Warrior Basin continues to be an active area for development of coalbed methane in spite of the expiration of the Section 29 tight gas tax credit. The majority of the successful wells have been in areas with relatively high permeability, with mixed results in low permeability areas. A study was initiated in late 1995 to determine if stimulation results could be improved in these areas by implementing specific optimization procedures for each of the coal groups. The optimization process involved extensive prefrac formation evaluation, injection/falloff testing, in-situ stress testing, fracture modeling using a P-3D simulator, perforating small intervals with 45 degree phased to minimize multiple fractures and tortuosity, intense quality control onsite prior to and during the jobs, estimation of spurt loss by pumping dual minifracture treatments, fracture height control by limiting rate and viscosity, real-time P-3D modeling of minifrac and main frac treatments to obtain tip screenouts, radioactive tracing of individual fluid and proppant stages with time-lapse monitoring, and postfrac history matching of job results. The real-time fracture modeling involved monitoring bottomhole pressures using a live annulus after comparison to data from a remote telemetry system and a quartz gauge on the initial well. Several practical innovations were developed during the study that will aid in designing the optimum treatment for each well.
The completion optimization tools and correlations developed during the project:
Development of a relationship between neutron log porosity and in-situ stresses, enabling the development of realistic in-situ stress profiles using only a basic log suite. The validity of the stress profiles was supported with real-time bottomhole pressure data during the treatments.
Verification that excessive downward proppant growth can occur when in-situ stress barriers are stronger above the perforations than below the perforated interval. This problem was most severe when thin fluids were used, and tip screenout treatments did not provide a universal solution. The initial downward growth was verified with multiple isotope tracer surveys, and subsequent further downward growth was verified with time-lapsed tracer surveys.
Verification that multiple zones cannot be effectively treated using limited entry techniques. Radioactive tracers confirmed that formation mechanical properties have a significantly greater effect on proppant placement than the allocation of perforations.
Development of reasonable estimates for spurt loss and total fluid loss coefficients for a 30 lb linear gel system in a moderate permeability environment. Spurt loss was obtained from two sequential minifracture treatments using a smaller treatment on the second job and evaluating the pressure decline of both treatments. The total fluid loss coefficient (Ct) was obtained on each well by using a small volume fresh water pump-in test at the beginning of each job, and excellent results were obtained in using this Ct estimate for the main job where 30 lb linear gel was pumped with a nitrogen assist.
Identification of a relationship between the magnitude of near-wellbore tortuosity pressure drop measured with the live annulus and proppant placement success.