Real-Time Formation-Face-Pressure Analysis Improves Acid-Fracturing Operations
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Knowledge of formation-face pressures is critical to the success of hydraulic-fracturing treatments, especially in multistage and multiple-cluster-type horizontal-well completions. This knowledge can help in evaluating the effectiveness of acid-fracturing treatments in dynamic mode and also can enable and improve real-time decision-making during treatment execution. This paper contains a detailed discussion of methods and a software tool that has been developed to generate information that predicts formation-face pressures in real time with the help of live bottomhole-pressure data.
Acid-fracturing treatments combine the basic principles of hydraulic fracturing and acid-reaction kinetics to stimulate acid-soluble formations. Unlike propped fracturing treatments, in which a viscous pad is followed by slurry-laden fluid that assists in keeping the fracture propped open to create a conductive flow path for formation fluids, the fracture conductivity in acid fracturing is generated by dissolving the formation face to create an uneven surface on the fracture walls.
Multiple pad and acid stages are pumped during the course of the treatment to create a high-conductivity fracture. The acid stage then fingers through the pad and the open fracture to create differentially etched patterns on the fracture walls. The uneven, etched surfaces prevent the fracture from closing completely once the hydraulic pressure is removed, leading to a highly conductive pathway to enhance hydrocarbon flow.
Once the acid contacts the formation, the bottomhole treating pressure recedes quickly, and, as acid spending continues, the effective bottomhole pressures can fall below fracture-closure stresses. Not all loss of pressure is a result of acid spending alone. The fluid fingering among fluids of highly contrasting viscosities also can result in such drops. However, the stress relief that results from acid spending dominates this behavior.
Pumping additional acid in these cases only results in near-well acid or mineral reactions, which do not extend fracture penetration and may increase wellbore-integrity risks. Unfortunately, without a good estimation of effective fracture pressures, determination of when the fracture-treatment pressure dips from above closure stress to below closure stress is difficult. Determination of this critical transition was a primary driver of the study that led to the creation of a tool that can assist in determining the real-time treating pressures at the entry of the fracture during stimulation treatments. Several acid-fracturing treatments were analyzed using the tool, leading to important conclusions related to fracture-propagation modes, acid-exposure times, and effectiveness of given acid types.
The majority of the horizontal wells considered for this study were drilled and completed in the North Sea with permanent bottomhole pressure gauges that enabled constant monitoring of well pressures. The software tool described in the complete paper uses a combination of treatment data such as surface pressure, fluid density, injection rates, type of fluid, wellbore description, gauge depth, wellbore deviation, and bottomhole pressures to generate formation-face pressures immediately outside the casing at active perforation depth. The tool performs the calculations as the treatment is being pumped, thus providing a dynamic array of several important parameters. It can also evaluate the treatment after it has been executed. The results of the study had a direct influence on modification of treatment designs and pump schedules to optimize treatment outcomes.
Methods, Procedures, and Process
Much of the complete paper is devoted to the presentation and discussion of calculations. These include pressure-related calculations in fracture-stimulation treatments, formation-face-pressure calculations, and the significance of bottomhole pressure. A variety of output data from the formation-face-pressure calculator tool is presented in Table 2 of the complete paper.
Failsafe features of the software tool are discussed. Because of the dynamic nature of the tool, the output solution is expected to have occasional errors. The source of the errors is in cases where the friction pressure gradients obtained while generating a dynamic-fluid-friction library are required to be extrapolated outside the range for which they were determined. Extrapolation is common, given the continual rate changes necessary to maintain surface treating pressures within safe operating limits while maximizing the injection rates to offset continuously the increasing leakoffs from acid spending. Also, if a change in tubing or casing diameter occurs below the pressure-gauge depth, the frictional pressure curves must be adjusted accordingly for constriction or expansion. Global safeguards include the following:
- Measured treating pressure (or surface pressure) is compared with the simulated surface pressure that is calculated using the derived tubular friction pressures as a quality check.
- Total measured tubing or casing friction above the pressure gauge is continuously compared with the calculated above-gauge friction pressure. If a discrepancy is found, the program loops back to rectify the error and calculations are run again. The corrected value is passed on to the tubular-goods frictional-pressure library.
The authors present several examples that cover both acid and propped fracturing cases. The formation-face pressure, especially in acid-fracturing treatments, clearly showed that, for certain legacy treatment designs, the majority of acid stages resulted in the formation-face pressures falling below fracture-closure pressure. However, changes made to the pump schedules thereafter helped minimize pumping time spent below fracture pressures. The authors state that having the data available in real time also allows the user to make on-the-fly decisions to prevent excessive acid volumes or, in some cases, even extend the acid volume if there are indications that more acid may be required for effective stimulation.
- Formation-face pressure is an important parameter in acid-fracturing and propped fracturing treatments; it drives the fracture geometry and can assist in treatment optimization.
- A tool to calculate formation-face pressure was developed and deployed to assist in understanding acid-fracturing stimulation in the field.
- On the basis of the observations and details from a study encompassing several treatments, the legacy pump schedules were replaced with more-effective and smaller treatments.
- Treatment volumes were significantly reduced in new acid-fracturing designs with little effect on well performance.
- Real-time ability of the tool allowed decision-making in the field when the treatments were still being pumped, ultimately improving operational efficiency.
- The tool aids in routine quality checks on fluid and additives being pumped because it relays pertinent information in real time.
- Analysis of propped fracturing treatments post-stimulation has helped in analysis of treatments (especially if effective bottomhole treating pressures were higher than overburden) and of their effect on well performance.
- Slurry friction was parsed from bottomhole-pressure data to indicate the correct net pressure trends in several propped fracturing treatments. This also indicated the extent of tubular- and perforation-related frictional pressure losses, especially at higher proppant concentrations.
- Plans exist to extend the use of tool in propped fracturing treatments.
For a limited time, the complete paper SPE 194351 is free to SPE members.
Real-Time Formation-Face-Pressure Analysis Improves Acid-Fracturing Operations
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19 June 2019