Over the last several years, the ability to perform accurate quantitative formation evaluation in high angle and horizontal wells has been recognized as high priority for major operators. The Logging While Drilling (LWD) has witnessed a revolution in technology in recent years. These new LWD tools have gained recognition in geosteering and well placement operations.
The need for advanced formation evaluation is more critical for carbonate reservoir where the complex pore structure will play a big role in fluid mobility. Formation mobility in carbonate reservoirs has been always an important output of any formation evaluation.
The South Fuwaris Field is located in the Partitioned Zone between Kuwait and Saudi Arabia. The Lower Cretaceous Ratawi reservoirs were discovered in 1961, and production commenced in 1963. There are two major reservoirs – the Ratawi Limestone, which is predominantly developed by vertical wells, and the Ratawi Oolite, which is exclusively developed by horizontal wells. The reservoirs comprise low-moderate permeability limestone. Interpretation is complicated by the existence of microporosity, mesoporosity and macroporosity. Understanding of pore type distribution would be of value is the placement of future Ratawi Oolite horizontal wells.
The LWD resistivity was used in this field as standard resistivity tool (Laterolog type) to determine the true formation resistivity. In addition, due to the fact that LWD tools measure while rotating, several resistivity images with different depth of investigation are also available in real time and recorded mode.
In this case, the LWD resistivity imaging was used to study azimuthal formation porosity distribution in order to quantify the different porosity portions (Primary / Secondary). Following that, an empirical equation was used to determine formation permeability profile. This profile can be further improved when calibrated with core data or formation pressure survey while drilling.