Matrix Acidizing, What Have We Missed: A Case Study of a Large Middle East Carbonate Field
Authors: Mudaliar Kokulananda , Aman Shah Muhammad Kamal , Salleh Zaimi 
Published: February 2020

Objectives/Scope: The paper is a case study of the impact of acidizing in a carbonate formation. The paper shares the operator’s observations and the approach taken to investigate the matter. The operator’s success in improving the acidizing approach is also discussed in detail. The paper also briefly touches on the effect of acidizing towards water flooding management and steps taken to manage the risk.

Methods, Procedures, Process: The study involves investigating behind casing communication post acidizing of a large middle east heterogeneous carbonate field where water injection is expected to commence soon. The field of interest has a reservoir thickness of 90-100m with vertical variation of permeability (10 md to 150 md). To achieve effective acidizing treatment and to provide flexibility during water injection operation, the wells were completed as single selective producers. The high permeability zones were isolated using packers with 8 meters separation. All wells completed were stimulated to create wormholes that can bypass the damage area and generate conductive flow paths, thus improving the well productivity. To improve production in the low permeability zones, a higher acid concentration was applied. During production test, it was found that the production rates and well parameters were the same when the zones tested commingled and individually. This triggered further investigation on the impact of matrix acidizing on the reservoir of interest. As part of the investigation multiple surveys were run to determine the acid profile as well as the zonal contributions post stimulation. In addition, simulation study and core flood test were conducted to investigate the acid reactivity and wormhole effect.

Results, Observations, Conclusions: Post evaluation it was concluded that matrix acidizing has potentially led to communication behind casing as a result of acid etching through the formation creating channels between the high permeability zone and low permeability zone. It is believed that the higher acid concentration which was used in the upper zone has reacted with the formation and created wormholes around the wellbore with gravity effect having exacerbated the situation further. To rectify the issue for upcoming wells the operator has relooked at the acidizing methodology and has successfully improve the acidizing recipe. However, the communication behind casing for existing wells still remains and this poses a high risk to water flooding management. The operator has taken the opportunity to relook at the water injection management scheme and accordingly made necessary changes to address the issue.

Novel/Additive Information: As part of the investigation, distributed temperature survey with fibre optic enabled coiled tubing was run. The technology was able to provide a continuous temperature profile along the well. Since the fibre optic was placed in the coil tubing, the temperature changes were able to be monitored both while pumping and during shut-in. The running of such a tool was new within the region with the operator being one of the first to employ the tool.