Decision Criterion for Stimulation Method: Proppant Fracturing, Acid Fracturing, or Matrix Acidizing?

Abstract

Most oil and gas wells are stimulated. In unconventional reservoirs, proppant fracturing is the preferred stimulation technique. However, in higher permeability conventional carbonate reservoirs, matrix acidizing and acid fracturing can also result in stimulated wells, usually with lower risk and cost. This article presents a decision criterion for the preferrable stimulation technique considering a combination of parameters, including permeability, stress, geomechanical properties, mineralogy, injection rate, proppant, fracturing fluid, and acid concentrations and volumes. This article is an extension of a previous study, in which a decision criterion was presented comparing only acid stimulation methods - matrix acidizing and acid fracturing. The present study developed a similar methodology to include proppant hydraulic fracturing in the analysis. The proposed methodology was developed using analytical derivations and validated using numerical simulators for both stimulation operations and production. The results were also validated and adjusted using field results and operational constraints. The final decision criterion is analytical and simple enough to be used in initial phases of conceptual design using a spreadsheet. The developed decision criterion was implemented in a simple software and has been used in conceptual design phase for several projects in offshore Brazil carbonates for a few years. It is especially useful in early conceptual design for two main reasons: (1) in this early stage there is usually not enough information available to justify using more complex and detailed numerical simulators, and (2) the well completions are often defined in this early stage, which requires a predefinition of the stimulation technique. Due to lower cost, risk, and simpler operations, acid stimulation methods are usually preferred when they yield similar results. Matrix acidizing is simpler and less risky than acid fracturing, which in turn is simpler and less risky than proppant fracturing. Agreeing with field observations, the presented calculation method shows that in low confining stress and hard rocks, acid fracturing can yield better results than matrix acidizing. However, acid fracturing is less effective in high permeability, high confining stress, or soft rocks. Proppant fracturing can usually result in more stimulated wells than the acid stimulation techniques in low permeabilities, but not in high permeabilities - despite higher cost and operational risk. This calculation method presents an agile quantitative estimate of the gain in productivity index that can be achieved by each technique, allowing an engineered quantitative decision for the stimulation method and corresponding well completion. It was observed that, although there is a good collection of stimulation models and software available, this simple method and its analytical equations provide insight and agility that cannot be replaced by software.