During a fracturing treatment, bottom hole fracturing pressure is used to evaluate the progress of the job. Direct measurement of bottom hole pressure is difficult. Surface treating pressure is commonly used to estimate bottom hole pressure, and must be corrected for fluid hydrostatics and friction losses in the tubing and perforations. This paper will illustrate the errors that can be made in fracturing treatment evaluation if proppant effects on fluid friction are not properly accounted for.
Real-time fracturing diagnostics using surface pressure require that accurate estimations be made for hydrostatic changes and friction losses to calculate the bottom hole fracturing pressure. During a fracturing treatment, the calculated bottom hole pressure is used to determine whether or not a well might be screening out. Early prediction of screen outs helps avoid situations where surface pressure increases above the designed pressure limits of the casing and surface equipment.
As part of a fracturing project in Lost Hills, California, a large number of older wells were selected for recompletion. These wells suffered from a variety of casing integrity problems related to subsidence and earthquake damage. Fracturing treatments in such wells could only be pumped down tubing.
It was well known from casing treatments what the expected pumping pressures should be, given the anticipated increase in friction losses in the tubing. In several cases, the calculated bottom hole pressure increased rapidly during the sand stages as if a screen out was in progress. However, when the well was flushed, the pressure would drop back to expected levels. There were no problems being encountered that would force early termination of the treatment except higher than expected surface pressure.
These early job terminations resulted not only in incomplete stimulation of the zone being treated, but also in increased costs and logistics problems. Excess sand and water had to be removed from location before frac tanks and bulk sand storage vehicles could be moved to the next location.