Study on the Influencing Factors of Matrix Acidification Effect in Carbonate Reservoir
Authors: Zhang L. , Xu H. , Zhao K. , Wang J. 
Published: August 2024

ABSTRACT:
Tight porous carbonate reservoirs cannot maintain high production for a long period of time by conventional depressurization recovery. Through the analysis of the physical properties of the four small layers divided vertically in the carbonate M reservoir of a block in Iraq, the MD and ME layers suitable for acidification are optimized. A radial acidification model was established by Comsol software, and the effects of acid pump rate and acidizing fluid volume on reservoir acidification were simulated and analyzed. The results show that: when the MD and ME layers select the appropriate acid pump rate of 1.4 bpm and 1.0 bpm respectively, the acid-etched wormhole can be effectively formed to improve the reservoir permeability. At a reasonable pump rate, when the volume of acidizing fluid is less, acidification only acts on a small area near the wellbore; with the increasing volume of acidizing fluid, the acid etching wormhole is formed. The volume of acidizing fluid continues to increase, the inlet pressure gradually decreases. When the pressure difference between the inlet and outlet cannot meet the established flow rate, the wormhole will stop expanding. The research provides a reference direction for the optimization of acidification process in low porosity and low permeability carbonate reservoirs.

1. INTRODUCTION
The carbonate M reservoir in a block of Iraq belongs to the tight porous carbonate reservoir, including four production layers of MB, MC, MD and ME. The net production layer thickness of the reservoir is more than 200 m. The mineral content is mainly calcite, higher than 93%, mixed with a small amount of clay minerals. The pores are mainly composed of moldic pores and intergranular pores, mixed with dissolution pores and a small amount of micro fractures. It has the characteristics of strong heterogeneity, low permeability and uneven porosity distribution, and it is difficult to increase production. The pore throat size and its proportion affect the reservoir and seepage capacity of the reservoir (Qiu, 2013; Ge, 2020). In general, the larger the radius of the connected pore throat and the higher the proportion of the rock, the better the reservoir category, on the contrary, the smaller the radius of the pore throat and the higher the proportion of the rock, the worse the category (Zhao, 2024). For carbonate reservoirs with strong heterogeneity, pore types are diverse and combination forms are complex. Connectivity can be improved by communicating reservoir pores, thereby improving reservoir porosity and permeability characteristics to increase oil well production. Matrix acidification is one of the important measures for carbonate reservoir reconstruction (Hoefner, 1989; Fredd, 1997; Daccord, 1989; Golfier, 2001; Wang, 1993; Frick, 1994; Buijse, 1998). The acidification process was carried out to increase the production of M reservoir before the oil well was put into production. However, due to the small strength of the corresponding acidification measures, it belongs to the near-well reservoir plugging removal measures, so the oil well production decreases rapidly. In order to achieve long-term high yield/stable yield, it is necessary to optimize the production process according to different reservoir characteristics, so as to improve the development efficiency. This study comprehensively analyzed the reservoir physical properties of the four production layers of the M reservoir, and selected the MD and ME reservoirs that are most suitable for acidification. According to the actual reservoir parameters of the production layer, the radial flow model of wormhole expansion is established, and the relationship between the acid injection rate and the injection volume and the acidification effect is studied and analyzed, which provides a reference optimization direction for the stimulation and transformation of similar reservoirs.