Analysis of Critical Operational and Reservoir Parameters in Carbonate Acidizing Design

Abstract

This work employs an integrated analysis method to quantify the impact of operational parameters and subsurface conditions on the acid stimulation efficiency of a carbonate reservoir. Examined parameters include reservoir type, completion design (initial skin), reservoir thickness, reservoir liquid properties (viscosity and compressibility), reservoir temperature, reservoir heterogeneity, acid type, acid volume, injection rate, and diversion technique.

In general, a combination of operational parameters and local geological conditions control the efficiency of carbonate acidizing. The operational parameters include acid type, injection volume, injection rate, and diversion technique. Optimizing these parameters maximizes stimulation efficiency and consequently hydrocarbon recovery. However, optimization needs to be evaluated within the context of local geology and subsurface conditions. For instance, the presence of natural fracture swarms or a high permeability zone along a limited length of the wellbore can present acid placement challenges that only advanced analysis can carefully address.

In our study, an integrated wellbore and reservoir model is used to simulate carbonate reservoir acidizing. The model first simulates the movement of stimulation fluid in a wellbore and couples it with transient reservoir flow. This analysis provides the distribution of reactive fluid along the well and calculates dynamics of skin evolution of a wellbore during stimulation. The model simulates the local change of injectivity and the interactions of different reservoir zones along the completed length of the well during stimulation time.

The analyzed results show that the interactions between wellbore flow and reservoir flow along the completed length of the wellbore is a dominant governing factor for stimulation efficiencies. It shows that at a limited volume of acid or low injection rate, most of the acid stimulates the upper sections of the reservoir while inadequate amounts of acid reach the bottom of the reservoir. The results show that the reservoir porosity system (i.e. porosity and permeability), pay-zone height, initial skin (i.e. completion design), reservoir temperature, and heterogeneity have governing effects on the optimum acid volume and injection rate. The reservoir condition might impose a wasteful acid volume and injection rate for a bullheaded treatment that can be optimized further with diversion technology. The latter situation being important in optimizing the volume of acid needed to efficiently stimulate the reservoir to obtain higher production.