A New Technique to Predict In Situ Stress Increment Due to Biowaste Slurry Injection Into a Sandstone Formation

Abstract

Underground injection of slurry in 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. 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. The objective in this study 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 fracture closure. The new technique predicts the fracture closure pressure from the instantaneous shut-in pressure (ISIP) and the injection formation petrophysical/mechanical properties including porosity, permeability, overburden stress, formation pore pressure, Young's modulus, and Poisson's ratio. Actual injection pressure data from a biosolids injector have been used to validate the new predictive technique. 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 injection and stress shadow analysis. Furthermore, the work shows that the injection 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.

Biomass slurry fracture injection as a potential low-cost negative emissions technology

“…A cyclic injection pattern is employed in which a 10-12 h injection cycle is preceded and followed by a brief clean water injection and an about 12-hour shut-in period. During this shut-in period, the injected water bleeds through the formation, reducing the pressure [12] to below the fracture pressure, and the fractures seal [13]. The process then reoccurs each day.…”

“…This lifespan is due to the eventual plugging of the formation fractures and increasing formation pressures [50] but is particularly uncertain due to the limited experience with the injection of biogenic slurries into underground formations. Kholy et al [13] modeled the total storage capacity of a single well in a high permeability formation and found total storage capacity to be on the order of 900 000 tonnes of solid material, not counting any biological degradation. In our model, each well receives 312 500 tonnes of solids over its 5 year lifetime.…”