The Real-Time Calculation of Accurate Bottomhole Fracturing Pressure From Surface Measurements Using Measured Pressures as a Base

Several papers written since 1978 have addressed the inportance of an accurate knowledge of bottom hole fracturing pressure. Nolte and Smith¹ in 1979 focused industry attention on the value of this information in predicting fracturing conditions. By the proper interpretation of the slope of log net fracture pressure versus log time, they showed that normal extension with confined height, fracture height growth with extension, loss of fluid loss control due to hairline fracture openings, and runaway growth of fracture height could be interpreted for fractures conforming to the fracture geometry of Perkins and Kern² and Nordgren³. Nolte⁴, in another paper, demonstrated that post-fracturing pressure decline data could be used to predict certain other fracturing parameters such as fracture height, leak-off coefficient, length, width, and closure time. Papers dealing with this technique include those of Veatch and Crowell⁵, Schlottman, et al⁶, Dobkins⁷, Smith⁸ and Smith⁹. Nolte, in 1982, wrote an excellent paper summarizing these techniques in SPE paper 10911, "Fracture Design Considerations Based on Pressure Analysis."¹⁰ Novotny¹¹ also proposed a fracture closure model based on fracture geometry and leak off. Erdle¹², et al, discussed the results of an experimental program conducted in Peru that addressed the same topics.

The authors here will not attempt to review or expand on the interpretive techniques given in the literature cited above. This approach has been generally accepted by the industry and dealt with in an excellent manner by the authors listed.

In the cited literature a reference string, either the tubing or the tubing casing annulus, was available to act as a monitor for bottom hole pressure behavior, that is the sum of the indicated surface pressure plus the pressure exerted by the hydrostatic head in the reference string is an accurate reflection of the true bottom hole fracturing pressure. The reference string, while desirable in terms of pressure measurement, presents some problems from both an operational and economic standpoint. In many cases the strength of the tubular goods is insufficient to allow the well to be fractured at meaningful rates with a reference string in the well. For higher pressured wells, treatment down tubing without a packer is many times impossible without casing rupture. In addition, unless the well can be placed in its final production configuration, the expense of tubing trips must also be borne. Where annular treatments are performed there is concern that the presence of the tubing may contribute significantly to shear degradation of the crosslinked gels and that injection rates using this configuration may be limited.¹⁰