Reinjection is one of the most important methods to dispose fluid associated with oil and natural gas production. Disposed fluids include produced water, hydraulic fracture flow back fluids, and drilling mud fluids. Several formation damage mechanisms are associated with the injection including damage due to filter cake formed at the formation face, bacteria activity, fluid incompatibility, free gas content, and clay activation. Fractured injection is typically preferred over matrix injection because a hydraulic fracture will enhance the well injectivity and extend the well life. In a given formation, the fracture dimensions change with different injection flow rates due to the change in injection pressures. Also, for a given flow rate, the skin factor varies with time due to the fracture propagation. In this study, well test and injection history data of a class II disposal well in south Texas were used to develop an equation that correlates the skin factor to the injection flow rate and injection time. The results show that the skin factor decreases with time logarithmically as the fracture propagates. At higher injection flow rates, the skin factor achieved is lower due to the larger fracture dimensions that are developed at higher injection flow rates. The equations developed in this study can be applied for any water injector after calibrating the required coefficients using injection step rate test (SRT) data.

In Dallas Fort Worth (DFW), sewage is treated with a combination of anaerobic digestion, effluent filtration and lime stabilization to create biosolids which are then composted, landfilled, or land applied. The current treatment procedure has certain concerns including emissions or accumulation of odors, pathogens, nutrients, metals, and pharmaceutical products.<br/> An alternative method, the Slurry Injection technique, enables the digestion of biosolids in the deep earth and can replace the current practice of wastewater treatment or disposal in a much more environmentally friendly and cost-efficient manner. By completely sequestering methane and CO2 into deep geologic formations which are produced as biosolids breakdown, reduces the greenhouse gas emissions and enables the operator to create greenhouse gas emission offset credits which can be marketed to offset the operating costs.<br/> The economic, environmental, and technical aspects of building a new biosolids slurry injection facility in DFW, includes both the surface construction requirements as well as the subsurface strata evaluation for containment assurance. For the subsurface aspects, a geomechanical and stress analysis is performed on the Atoka formation (near the city of Fort Worth) and it confirms a confining layer above and below the injection zone to keep the waste contained for permanent storage.

Summary During hydraulic-fracturing operations, conventional pressure-falloff analyses (G-function, square root of time, and other diagnostic plots) are the main methods for estimating fracture-closure pressure. However, there are situations when it is not practical to determine the fracture-closure pressure using these analyses. These conditions occur when closure time is long, such as in mini-fracture tests in very tight formations, or in slurry-waste-injection applications where the injected waste forms impermeable filter cake on the fracture faces that delays fracture closure because of slower liquid leakoff into the formation. In these situations, applying the conventional analyses could require several days of well shut-in to collect enough pressure-falloff data during which the fracture closure can be detected. The objective of the present study is to attempt to correlate the fracture-closure pressure to the early-time falloff data using the field-measured instantaneous shut-in pressure (ISIP) and the petrophysical/mechanical properties of the injection formation. A study of the injection-pressure history of many injection wells with multiple hydraulic fractures in a variety of rock lithologies shows a relationship between the fracture-closure pressure and the ISIP. An empirical equation is proposed in this study to calculate the fracture-closure pressure as a function of the ISIP and the injection-formation rock properties. Such rock properties include formation permeability, formation porosity, initial pore pressure, overburden stress, formation Poisson's ratio, and Young's modulus. The empirical equation was developed using data obtained from geomechanical models and the core analysis of a wide range of injection horizons with different lithology types of sandstone, carbonate, and tight sandstone. The empirical equation was validated using different case studies by comparing the measured fracture-closure-pressure values with those predicted by using the developed empirical equation. In all cases, the new method predicted the fracture-closure pressure with a relative error of less than 6%. The new empirical equation predicts the fracture-closure pressure using a single point of falloff-pressure data, the ISIP, without the need to conduct a conventional fracture-closure analysis. This allows the operator to avoid having to collect pressure data between shut-in and the time when the actual fracture closure occurs, which can take several days in highly damaged and/or very tight formations. Moreover, in operations with multiple-batch injection events into the same interval/perforations, as is often the case in cuttings/slurry-injection operations, the trends in closure-pressure evolution can be tracked even if the fracture is never allowed to close.

Oilfields produce huge amount of waste on daily basis such as drilling mud, tank bottoms, completion fluids, well treatment chemicals, dirty water and produced saltwater. These waste types represent a real challenge to the surrounding environment especially when the oilfield is located within a sensitive environment as in the Western Desert where there are natural reserves and fresh water aquifers. Waste slurry injection has proven to be an economic, environmentally friendly technique to achieve zero waste discharge on the surface over the past years. This technique involves creating a hydraulic fracture in a deep, subsurface, non-hydrocarbon bearing formation which acts as a storage domain to the injected slurrified waste. The objective of this study is to evaluate the feasibility of waste slurry injection in an oil prospect located in the Western Desert. The evaluation includes assessing the subsurface geology, recognizing the possible candidate injection formation(s), and designing the optimum injection parameters. Both geological and petrophysical data have been used to create the geomechanical earth model for an oil prospect located at Farafra oasis in the Western Desert. This model defines the mechanical properties, pore pressure, and in-situ stresses of the different subsurface formations. Afterwards, a fully 3D fracture simulator has been used to simulate the fracture growth within the candidate injection zone at different injection scenarios. Additionally, the fracture simulator has assessed the containment of the created fracture within the candidate injection formation(s) due to the presence of stress barriers above and below the formation. Finally, the formation disposal capacity has been calculated for each of the injection scenarios using a stress increment model. The geomechanical earth model shows that there is a good candidate injection zone which is upper/lower bounded by stress barriers. More importantly, it is located deeper than the local fresh water aquifer and thus no contamination is expected to the fresh ground water. In addition, the possible candidate is not a hydrocarbon bearing formation. A 3D fracture simulator has been used to determine the optimum injection parameters such as: the injection flow rate, the volumetric solids concentration, the slurry rheology and the injection batch duration. These optimum parameters are defined to minimize the stress increment rate over the well life, which ensure the highest disposal capacity and to contain the fracture within the candidate injection formation. Guidelines to conduct waste slurry injection technique in a new oil prospect that is located within a sensitive environment as in the Western desert are presented in this study. Also, the study highlights that this technique is economic for disposal of the different oilfield waste types in an environmentally friendly fashion.

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.