Optimizing Limestone Acidizing Treatments in Perforated Horizontal Completions by Implementing a Physics-Based Tool
Authors: Perez Oswaldo , Fragachan Francisco , Babey Andrew 
Published: November 2019

To increase the likelihood of success of acid stimulation in limestone reservoirs, the treatment has to evenly cover the desired zone to allow controlled reaction rates that can result in a uniform conductivity pattern, or wormholes development radially across the pay zone. To achieve this ultimate goal, effective fluid diversion is required to reorient fluid path, from high to low injectivity areas. The selection of the right diversion technique is the key to obtaining successful stimulation results. Therefore, The objective of this work is to evaluate, and compare the stimulation efficiency of several diversion scenarios based on a highly reliable physics-based tool capable of simulating multiple completion types.

This work will be focused on two typical diversion methods applicable to perforated completions, such as: 1) ball sealers, and 2) bio-degradable particles. A coupled model that consists of wellbore and reservoir flow is used to simulate acid, and limestone rock interactions for each diversion method. The model simulates fluid hydraulics in the wellbore, couples it with transient reservoir flow, and accounts for the formation skin effects derived from each diversion technique. The model also considers the effect of induced wormholes generation and the created injection profile along the completed reservoir zone.

A horizontal well completion is presented to demonstrate the impact of each diversion approach in order to assess the effectiveness of a stimulation design. The most effective sensitivity combination of each diversion method is the focus of this work, and the treatment invasion distribution across the completed interval is compared to determine the best diversion approach. Different ball sealers geometries are considered to model irregular-shape perforation plugging efficiency and subsequent fluid diversion. The enhanced ball sealers model considers several physics parameters such as: inertial force, drag force, and ball-holding force along the wellbore during stimulation. On the other hand, the particulate diversion model includes an engineering model that is integrated into the wellbore-reservoir model to simulate the particle diversion.

The particulate diversion model is a binary system that consists of: (1) large particles agglomerate along the tapered path of wormhole, and perforations, and (2) small particles jamming effect to create a temporary sealed structure that reduces the permeability of flow path and builds a temporary filter cake on perforations that is capable of holding up necessary differential pressures to divert fluid to other low-injectivity zones.

The results show that the diversion efficiency depends basically on the length, perforationsconfiguration, and the reservoir heterogeneity. This case study demonstrates that particulate diversion offers the best alternative in terms of economic feasibility, and ease of application.

The current tool has the unique capability combined with an integrated approach to optimize diversion scenarios for matrix acidizing stimulation on limestone reservoirs to generate a more uniform wormhole pattern, avoiding fluid loss, and tapping into under-stimulated rock areas for production enhancement.