An analytical technique has been developed to consider fluid compressibility and temperature changes when making mini-frac analyses.
Mini-frac tests in which the effects of compressibility and changing temperatures are ignored can produce results that are as much as 75% in error. However, techniques for mini-frac analysis presented in current petroleum engineering literature are based on the assumption that fracturing fluid is incompressible and isothermal in nature.
This paper examines the effects that fluid compressibility and temperature changes have on mini-frac analysis by presenting field cases analyzed by the method developed. The analysis was performed using both PK and CZ models as presented in literature.
An example analysis using the PK model showed that when both temperature and compressibility effects are ignored, errors of 300% in calculated fluid loss coefficient and 75% in calculated fracture length are encountered. The same example using CZ models gave 200% and 33% errors in fluid loss coefficient and fracture length, respectively.
Correcting for temperature and compressibility effects is considered through the use of "effective pressure drop," or pressure drop the mini-frac would pressure drop," or pressure drop the mini-frac would have exhibited if ideal conditions had prevailed. Developing this concept allows the use of already developed type curves. The approach is analogous to pseudo pressure in well testing. When only pseudo pressure in well testing. When only compressibility effect is considered, only the magnitude and not the shape of AP versus dimensionless time graph is changed. Correcting for both temperature change and compressibility changed both shape and magnitude of the curve. It has been observed that a better match was obtained after such correction.
The introduction of mini-frac analysis in 1979 was a breakthrough in fracturing technology. it allowed field calculation of important parameters such as leakoff coefficient, fracture length and fracture width. Mini-frac analysis consists of performing a small fracturing treatment with little performing a small fracturing treatment with little or no proppant, then the well is shut in and pressure decline with time is monitored. Rate of decline of pressure with time depends on formation, fluid and pressure with time depends on formation, fluid and fracture parameters.
Nolte presented the original analysis for mini-frac tests. His analysis was based on Perkins and Kern geometry and on work presented by Nordgen. Later Nolte and Lee expanded this technique to include the CZ model with geometry developed by Daneshy and a radial model (commonly referred to as penny-shaped fracture). Lately Martin presented a penny-shaped fracture). Lately Martin presented a model considering an ellipsoidal fracture. Lee has developed a similar technique.
The basic approach for analysis of a mini-frac test as presented by Nolte, Lee and Martin is the same as originally developed by Nolte. The only difference is in utilizing an equation that relates pressure to fracture width. pressure to fracture width. Although mini-frac analysis presented a novel idea, it ran into trouble when it was applied to hot wells where the isothermal condition assumed in the original analysis was severely violated. Pressure rise was sometimes recorded. This led to development of work presented in this paper. Techniques presented for analysis of a mini-frac also assumed that presented for analysis of a mini-frac also assumed that fracturing fluid is incompressible, which may not be a good assumption, especially if foam is used as fracturing fluid.