Real-Time Formation-Face-Pressure Analysis Improves Acid-Fracturing Operations
Authors: Feder Judy 
Published: May 2019

This article, written by JPT Technology Editor Judy Feder, contains highlights of paper SPE 194351, “Real-Time Analysis of Formation-Face Pressures in Acid-Fracturing Treatments,” by Vibhas Pandey, SPE, and Robert Burton Jr., SPE, ConocoPhillips, and Kay Capps, Capsher Technology, prepared for the 2019 SPE Hydraulic Fracturing Technology Conference and Exhibition, The Woodlands, Texas, USA, 5–7 February. The paper has not been peer reviewed.
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.
Introduction
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.