Dhafer Al-Shehri

Wormhole Propagation in Tar During Matrix Acidizing of Carbonate Formations

Heavy, viscous oil deposits and tar have low o API gravities and occur as part of several oil formations. Unlike normal oil deposits, heavy oil and tar tend to contain more inorganic impurities and to be more sulfurous and aromatic. And as so, they tend to have different responses to acidizing fluids during matrix acidizing treatments. One fundamentally interesting phenomenon is the wormholing characteristics of acidizing tar formation. This paper discusses the effect of acid and its wormholing characteristic on tar and on carbonate rock that was saturated with crudes that have varying o API gravities. Experiments included acid flooding of core plugs that were saturated with different o API gravities. The extreme case included flooding the acid through tar saturated plugs. The wormholes were characterized by CT Scanning. Differential pressures, number and sizes of wormholes and breakthrough volumes were all measured for each experiment. The tests involved regular hydrochloric acid and emulsified acids. This study showed that regular and emulsified acids produced comparable wormhole penetration in tar. Tar formations were difficult to exhibit face dissolution even at extremely low injection rates. In general, it was noticed that penetration and, hence, benefit from emulsified acid is reduced when higher o API oil saturated the rock. The wormhole breakthrough volume in a rock saturated with intermediate oil was less than that of a rock saturated with condensate oil. Condensate might have allowed better diffusion of acid droplets to react with the rock. This work provided a fundamental investigation that can lead to development in producing these challenging prospects. In addition, these results are of special interest when long horizontal injectors or producers are placed within the tar zone of conventional oil reservoirs.

Utilizing NMR and Formation Pressure Testing While Drilling to Place Water Injectors Optimally in a Field in Saudi Arabia

The presence of tar or heavy oil that does not flow using conventional production technologies brings numerous challenges during field developments. Tar, acting as a permeability barrier, would often break flow or pressure communication from the aquifer to the oil zone. This results in inadequate pressure support, which is necessary for sustaining production levels and maximizing oil recovery. One of the key issues in developing a field with known tar mat accumulation is to optimally place injectors away from the tar. The problem becomes more complicated when the exact location of a tar mat is uncertain either laterally or vertically. Tar mats usually are neither flat nor uniform in thickness across a field. These uncertainties pose a challenge in planning wells especially water injectors. Detection of tar is critical for reservoir characterization, reserves calculation and well placement. Direct and indirect techniques are employed to detect tar including core analysis, well testing, wireline logging and Pyrolitic Oil Productivity Index (POPI). These measurements are good indicators of tar; however, the challenge is to identify the tar while drilling the well. Early detection requires the deployment of logging while drilling (LWD) technologies for real-time interpretation of data. In order to accurately identify tar in reservoir sections in real-time, integrating conventional LWD measurements with new technologies such as the slim hole Nuclear Magnetic Resonance (NMR) and the formation pressure measurements while drilling (FPWD) is necessary. This will allow for timely adjustment to the well path and prevent costly remedial actions This paper discusses successful real-time application of slim-hole NMR and FPWD technologies to detect tar and optimally place water injectors. This is demonstrated with two case studies involving extended reach power water injectors.