The fractured basement field in Yemen described in this paper is characterized by two types of fracturing: background fractures with a very low effective permeability of less than 0.001 md and fracture corridors with an effective permeability of up to several millidarcies. Except for some dissolution porosity related to fracture corridors, no significant matrix porosity is encountered (total porosity is only 1.15%). Approximately one-half of the oil in place is contained in the fracture corridors and one-half in the background fractures.Production from this field commenced in 2007. It is currently produced by depletion. Compositional grading has been observed in the 3,120-ft oil column. Despite the fact that the oil is close to bubblepoint pressure at the top of the reservoir, a moderate increase in gas/oil ratio (GOR) has been seen.Detailed studies using material balance and discrete-fracturenetwork (DFN) models revealed that the reason for the slow increase in GOR is the low permeability of the background fractures. The low permeability leads to viscous forces being dominant over gravity forces and, hence, limited gravity segregation of gas and oil.Because of the relatively small viscosity difference between the gas and the oil in this field ( o / g = 6.5), the gas mobility is not much higher than the oil mobility at low gas saturations. Hence, oil and gas are produced effectively from the background fractures into the fracture corridors and the reservoir pressure is not depleting as fast as in reservoirs with higher viscosity difference between gas and oil. This results in a more effective solution-gasdrive recovery mechanism than that expected for a conventional reservoir.A number of reservoir-management strategies have been investigated. The results indicate that the low permeability of the fracture corridors and very low permeability of the background fractures lead to low recovery factors of 14% for gas injection. However, the efficiency of solution-gas drive is higher than in conventional reservoirs.

The fractured basement field in Yemen described in this paper is characterised by two types of fracturing: background fractures with a very low effective permeability of less than 0.001 mD and fracture corridors with an effective permeability of up to 1 mD. Except for some dissolution porosity related to fracture corridors, no significant matrix porosity is encountered (total porosity is 1.15 % only). About half of the oil in place is contained in the fracture corridors and half in the background fractures.Production from this field commenced in 2006. It is currently produced by depletion. Compositional grading has been observed in the 3120 ft oil column. Despite the fact that the oil is at bubblepoint pressure at the top of the reservoir, a moderate increase in gas/oil ratio (GOR) has been seen.Detailed simulation studies revealed that the reason for the slow increase in GOR is the low permeability of the background fractures. The low permeability leads to viscous forces being dominant over gravity forces and hence almost no gravity segregation of gas and oil.Due to the relatively small viscosity difference of the gas and the oil in this field, the gas mobility is not much higher than the oil mobility at low gas saturations. Hence, oil and gas are produced effectively from the background fractures into the fracture corridors and the reservoir pressure is not depleting as fast as in reservoirs with higher viscosity difference of gas and oil.A number of reservoir management strategies have been investigated. The results indicate that the low permeability of the fracture corridors and very low permeability of the background fractures results in challenging conditions for increasing oil recovery by water or gas injection. However, the efficiency of depletion drive is higher than in conventional reservoirs.

This work addresses the challenging task of history matching a fractured basement reservoir. Habban field, south-western part of Yemen, produces from two different horizons: the Kuhlan sandstone overlaying the fractured basement. The fractured basement reservoir is characterized by two types of fracturing: background fractures with very low effective permeability of less than 0.001mD and fracture corridors with an effective permeability ranging from 0.01 up to 10 mD, and a total porosity of 1.3% only. Two sets of fracture corridors, with N-S and NW-SE mean orientation, contribute to production whereas background fractures act as storage feeding in those corridors. The large contrast in properties between Kuhlan and Basement adds-on further challenges: Kuhlan possesses good reservoir properties but moderate storage (~10m thick), whereas fractured basement has extremely poor reservoir properties but significant storage (~700m thick). Habban field has produced since end 2006 by depletion through 30 slanted wells. To optimally assess different production strategies, a simulation model was built. Dynamic data (MDT, PLT, BHP, production rates) were used to constrain the dynamic model on well by well basis. Due to this extremely challenging setting, a very high focus was put on a history matching process that integrates geology and reservoir simulation work to result into an enhanced understanding of field mechanisms. One main conclusion of this integrated history matching approach is that water comes to wells from neighborhood formations through main faults, explaining then the erratic water production (no crest to flank correlation). Second highlight concerns the major contribution of N-S main fractures. Finally understanding of the communication mechanism between basement and Kuhlan was enhanced, showing that despite basement poor properties, basement oil feeds in Kuhlan, as Kuhlan is produced. This synergized understanding of Habban mechanisms is a clear milestone for further well location planning and oil recovery optimization under uncertainty.

It is a considerable challenge to effectively develop hetero-geneous fractured reservoirs in complex structural settings. Here we present a case study of a fractured basement reservoir hosted in structural highs formed by fault-controlled blocks in a rift setting. A wide range of data were available to study the current reservoir. The seismic database includes a wide-azimuth data set, as well as an extensive set of derived attributes and structural interpretation. Azimuthal anisotropy, automatic fault detection, and rms amplitude proved to be most useful in the description of the fracture network, the fracture corridors, and a thin, permeable sandstone layer above the basement, respectively. An additional advantage of the wide-azimuth survey was that it provided an optimum illumination in complex structural settings such as the current case. To complete this large-scale data set, conventional log suites and image logs were acquired in most of the wells. Borehole image-log studies were conducted to interpret and recognize fracture features. Regarding dynamic information, flowmeter logs were acquired in addition to the production history. The main objectives of this integrated study were the creation of a conceptual model for well planning and to constrain the static and dynamic properties of the reservoir model.

No abstract