An Investigation of a High-Strength Proppant Tail-In at McAllen Ranch Field

Abstract

ABSTRACT Producing deep, geo-pressured, extremely tight gas reservoirs which have received massive hydraulic fracture treatment, creates severe crushing stresses on proppant placed in the fractures. Several high strength proppants are available on the market. While these proppants are superior to frac sand they are considerably more expensive. The question of the producer is one of pure economics. In some cases economics of a prospect may be marginal ; therefore, the operator considers alternative measures. One alternative is modification of the proppant schedule to place a regular strength proppant deep in the fracture and a high strength proppant next to the wellbore where closure stresses will be greatest. This paper discusses the evaluation of proppant placement in a massive hydraulic fracture treatment "tail-in" technique. The term "tail in" is used to describe the final proppant stage of a multi-stage fracture treatment. A new evaluation method was developed to determine placement of this tail-in stage. For this new method the leading stages of proppant laden fluid were tagged with a long half-life isotope, 74 days. The tail-in of proppant laden fluid was tagged with a short half-life isotope, 8 days. Comparison of calibrated radiation logs recorded before and immediately after fracture treatment define the placement of proppant tagged with both isotopes. A third radiation log, recorded at the appropriate time measures decreases in radiation of both isotopes. Processing data from these three logs in a mathematical model enables calculation of proppant placement during treatment. Four wells were treated in Forest Oil Corporation's McAllen Ranch Field, Hidalgo County, Texas. Of these four wells, one was treated with a "tail-in" technique and the other three with a 100% bauxite simulated "tail-in" technique. In these four wells, the proppant was tagged with two different radioactive isotopes to determine If the "tail-in" fraction was placed next to the wellbore. In all four wells, the evaluation indicated both proppants next to the wellbore. Factors causing this were diversion to different members during treatment and override of the leading proppant. Without proper placement of the bauxite "tail-in", extreme closure pressures near the wellbore would crush the regular strength proppant and loss of reserves would result. Post-frac buildups were performed on two wells which had been subjected to approximately the same closure pressure. The well with 100% bauxite had much greater fracture flow capacity, than did the well with a tail-in treatment. Experience gained from these four wells indicate that the "tail-in" technique of stimulation was not successful at McAllen Ranch Field.