Abstract

Underground injection of slurry in batches or cycles with shut-in periods allows fracture closure and pressure dissipation which in turn prevents pressure accumulation and injection pressure increase from batch to batch. The "G-function" technique is a well-known method for analyzing the pressure fall off data and has been used in monitoring the evolution of formation stress and to identify the fracture closure point after each injection batch. However, in many cases the accumulation of solids on the fracture faces slows down the leak off which can delay the fracture closure up to several days. Well shut-in for such a long time between the batches is impractical. The objective of this work is to develop a new predictive method to monitor the stress increment evolution when well shut-in time between injection batches is not sufficient to allow the fracture to close.

The new technique predicts the fracture closure pressure based on the knowledge of the instantaneous shut-in pressure (ISIP) and the injection formation petrophysical and mechanical properties including: porosity, permeability, overburden stress, formation pore pressure, Young's modulus, and Poisson's ratio. The injection pressure data from actual biosolids injection operations in Los Angeles, California has been used to validate the new predictive technique. The G-function analysis method was used to identify the fracture closure pressure in the early well life before solids accumulation on the fracture faces slowed the leak off rate. In later injection batches, solids accumulation did not allow fracture closure to occur during the well shut-in. Hence, the new technique was successfully used to build the stress increment profile of the injection formation.

During the early well life, the match between the predicted fracture closure pressure values and those obtained from the G-function analysis was excellent, with an absolute error of less than 1%. In later injection batches, the predicted stress increment profile shows a clear trend consistent with the mechanisms of slurry inj ection and stress shadow analysis. Furthermore, the work shows that the inj ection operational parameters such as injection flow rate, injected volume per batch, and the volumetric solids concentration have strong impact on the predicted maximum disposal capacity which is reached when the injection zone in-situ stress equalizes the upper barrier stress. In addition, the results show that the formation disposal capacity increases when the injection flow rate and the injected volume per batch increase.

The new technique helps in predicting the stress increment over time even when the well shut-in duration is shorter than the fracture closure time. As a result, safe injection operations can be conducted by assuring that stress increments are within allowable limits without extending the shut-in period after each injection. Another advantage of the technique is that it assists in optimization of the injection parameters to achieve the maximum possible injection capacity of the formation/well.

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