Further development of the fractured Natih field requires a good understanding of the historical contributions from gas/oil gravity drainage and water/oil displacement. A history match of the field has been made using a fractured reservoir simulator, which apart from dual porosity, can model dual permeability and block-block interaction. Several studies aimed at describing the reservoir in detail were required before simulation could start. The history match shows that gas/oil gravity drainage has been about twice as effective as water/oil displacement. This figure is in good agreement with the gas and water sweep data derived from recent gas saturation and water saturation measurements in the field. Prediction runs show that promoting gas/oil gravity drainage by Prediction runs show that promoting gas/oil gravity drainage by lowering the fracture oil rim is an attractive way to further develop the Natih field.
The Natih field, situated in the North of Oman (Fig. 1) is a domal structure, bounded to the North by a large reverse fault. The field measures some 10 km by 6 km (Fig. 2). The main reservoir is the Middle Cretaceous Natih formation which is composed of a series of regressive marine carbonate cycles. The field originally contained some 460 million m3 of 865 kg/m3 (32(0) API) oil. The field was discovered in 1963, brought on stream in late 1967, and has produced to date some 58 million m3 oil (12.5% of the oil originally in place). Both chalky and vuggy/rudist facies are recognized in the carbonate cycles. All facies have moderate to low matrix permeability (from less than 1 mD upto 20 mD) but are heavily fractured which strongly influences the fluid flow behaviour in the reservoir. A combination of water/oil and gas/oil displacements has taken place in the reservoir since the start of production: a period of depletion, followed by water injection, later production: a period of depletion, followed by water injection, later in combination with gas injection (Fig. 3). The field is currently produced from a fracture oil rim at an oil rate of around 3800 m3/d. produced from a fracture oil rim at an oil rate of around 3800 m3/d. In the second half of the 80's further development of the field required a much better understanding of the relative effectiveness of the displacement processes. More direct measurements and studies have been undertaken aimed at describing the reservoir in sufficient detail to allow for an integral history match.
The main reservoir of the Natih field is the Natih formation which is some 380 m thick and subdivided into vertically stacked members, of which A to E are oil bearing (Table 1). The overlying Fiqa shales act as a cap rock and extend to surface. Beneath the Natih formation are the Nahr Umr shales which seal the oil bearing Shuaiba formation. Division into subunits is largely based on relative position in a depositional cycle. A fully developed cycle contains shale at the base and grain-supported limestone at the top. The units have therefore variable reservoir quality, usually improving upward in a cycle. The A unit contained some 70% of the fields hydrocarbons originally in place, and forms the main development target. As a result of subaerial erosion the Natih A thins in a southwestern direction. Associated fresh water leaching has caused an increase in matrix porosities and permeabilities in the same direction. The Natih B unit consists of poor to non-reservoir rock. It contains organic material which is assumed immobile. Natih C comprises good reservoir rock with rudist found at the top of the C1 subunit. The Natih C contained about 20% of the oil originally in place.
The Natih formation is heavily fractured with an average fracture spacing of 1 m in the more permeable units. It contains predominantly NE-SW trending open fractures as detected in wells predominantly NE-SW trending open fractures as detected in wells from Formation Micro Scanner (FMS) studies and evaluation of palaeomagnetically oriented cores. palaeomagnetically oriented cores. P. 309