No abstract

Karst features in the upper part of Mishrif carbonate reservoir can commonly create substantial vugs, pores and fissures and, thus, increasing the porosity and enhancing reservoir connectivity and permeability, this features were formed normally as a result of the carbonate rocks dissolution under the action of meteoric waters during very long periods of emersion spanning from Late Cenomanian to Turonian (about 4.5 Million years). This study deals with the presence of the karst features in Zubair oil field, southern Iraq, where Karst features were observed in the center and northern parts of the field close to the top of Mishrif Formation in most of the cored wells. Characterization of that feature can be achieved from static data at wellbore scale from core, and logs (conventional and non-conventional), and dynamic data such as mud losses, well tests combined with production logs (PLT) in addition to Seismic data. Beside the positive and economic effect of Karst, there is negative effect which includes causing mud losses during drilling in Mishrif Formation because of low reservoir pore pressure, Therefore, acid soluble cement plugs were used to heal that losses to continue drilling.

Water injection is the main strategy to increase oil production and support reservoirs pressure in the X oilfield and almost any other oilfield. The available source for the water injection in X field is the Qarmat Ali river which is located 20km to the north. However, the amount of water from that river is insufficient. Therefore, alternative water sources were investigated and among them, it was identified the Nahr Umr aquifer as the most promising. Nahr Umr is a regional giant aquifer; it is composed of high porous high permeable sandstone at an average depth of 2900m. The analysis of Nahr Umr aquifer water showed that it has a very high salinity (240000 ppm) almost composed of sodium and calcium chlorides, free of Oxygen, no solid particles were observed, no quantities of dissolved H2S, and very low occurrence of bacteria owing to the high water salinity and high temperature of the aquifer. On other hand, aquifer water contains a significant amount of dissolved CO2 (5.5%) which can cause corrosion. Therefore, there is a need to treat that water to decrease CO2 or use high corrosion-resistant materials in the pipelines and water source wells.

Sedimentary rocks can be defined from others not only by their lithology, structures, fossil content, geometry and sedimentary but furthermore by their general response to the wireline logs. This study involves the determination of the properties of Upper Shale Member of Zubair Formation (Lower Cretaceous) in Rumaila oilfield by using several log interpretation software through wireline logging response of lithology. Many wells were chosen in Rumaila oilfield to know the lithology of Upper Shale Member, in addition to calculating permeability for Rumaila oilfield by using eight wells through Rumaila cross-section. The Gamma Ray log responses of clean sand range between 30 – 40 API, while it decreased in the case of silt and clay cemented sand grain. Sandstones are dominated in the south of Rumaila, this could increase the porosity and permeability of the rocks. While clay and shale are dominated in the north of Rumaila. Log responses don’t give a true representation for formation lithology if comparison with true core lithology by using grain size analysis.