Tar-mat zones have been characterized at the base of oil columns in some of Saudi Arabia carbonate reservoirs. These zones form a barrier which physically isolates the producing zones and the injection water. As a result of this barrier, the reservoir pressure will decrease, which will require other means to produce these wells. This paper addresses two strategies to overcome potential problems resulted from tarmat zones.
The first strategy was to drill a tunnel well. The tunnel well works as a conduit that connects the high pressure area to the low pressure area through the tar zone of the reservoir. Field data indicated that this well enhanced production rate of nearby wells and livened two dead wells.
The second technique to address tar-mat present in another field is to use peripheral water injection. Water injectors with extended reach (ER) were completed horizontally at the bottom of the producing zones and just above the tar-mat zones. Challenges were faced in drilling these wells in a populated area in the northern area of Saudi Arabia and being stratified, tar-mat was encountered in open-hole sections of these wells. Tar content varies inconsistently around these injectors and more near wellbore and obviously in the tar layer. This limits the injection rate of these injectors initially to as low as 600 BWD at 2,400 psig.
The low injectivity is attributed to the damage induced by the drilling mud (water-based mud) and the presence of tar and asphaltene in the injection zone. Acid stimulation treatments of these wells were needed to reach the target injection rate. A tailored acid treatment was designed to address these challengers. This treatment included pumping aromatic solvent, a regular acid, an emulsified acid and a viscoelastic surfactant-based acid for diversion.
Because of environmental concerns, high H2S content, the spent acid was not lifted following the acid treatments. However, the injectivity index of horizontal injectors increased by several folds.
The paper describes the efforts of a multi-disciplinary team to enhance the performance of these wells by using the latest technologies in drilling, logging, and well treatment.