Assessment of Halite-Cemented Reservoir Zones

Summary This paper describes the techniques used to identify the presence anddistribution of presence and distribution of halite-cemented layers in asandstone reservoir. The distribution of these layers in the wells was found bymatching the core data with two independent halite identifiers from the welllogs. Numerical well models were used to assess the dimensions and spatialdistribution of the halitecemented layers. Multiple simulation runs in whichthe spatial distribution, the dimensions, and the vertical permeability werevaried resulted in a permeability were varied resulted in a stochastic modelthat best matched the production history. Gas and water coning are retarded bythe halite-cemented layers if the perforations are properly located.perforations are properly located. Introduction The Azal field is located in the Marib AlJawf basin in the Republic of Yemen(Fig. 1). Of the 20 wells spudded in the field, 13 are oil-production wells andone is a gas-injection well. Production started in May 1988. The reserves areestimated to be 142 × 10(6) bbl [22.6 × 10(6) m3] of oil and 568 × 10(9) scf[16.1 × 10(9) std m3] of gas. The Azal field is an extension northeast of thelarger Alif field, from which it is separated by a half-graben structure. Thefield is 5.6 miles [9 km] long and 0.9 mile [1.5 km] wide. The depth of the topof the reservoir is 6,625 ft [2019 m], which is 3,425 ft [1044 m] below meansea level (MSL). The oil column, with a 39 degrees API [0.830g/cm3]-specific-gravity oil, has an average thickness of 80 ft [24.4 m]. The maximumga-column thickness is 240 ft [73.2 m]. The gas has a high liquid content, andthe underlying aquifer is oversaturated with salt. The Alif field, which hascharacteristics similar to those of the Azal field but a thicker oil column, started production in March 1986. After some time, salt deposition clogged anumber of wells and production facilities. Special measures were necessary toprevent production loss. The salt deposition may have resulted from pressureand temperature changes in the brine produced concurrently with the oil as aresult of water coning. No halite was observed during the formation evaluation;however, no special attention was paid to this aspect at that time. Indicationsof balite were found in the Azal field formation. A detailed core analysisrevealed the existence of diagenetic, postcompactional halite in the Alifmember postcompactional halite in the Alif member (the main hydrocarbon-bearingunit). Furthermore, the production history of the Azal field showed gas andwater production rates lower than those anticipated from the coningcalculations, which indicated that the flow path to the well was prolonged byflow path to the well was prolonged by flow barriers. In this very cleansandstone reservoir with a minor amount of diagenetic minerals, flow barrierscould well be made up of halite-cemented layers. The central issue addressed inthis paper is whether the balitelcemented layers observed in the reservoir willinfluence salt deposition in the wells and production facilities of the Azalfield by retarding water and thus salt flow through the oil column to thewells. This information is important for the planning of well locations andperforation intervals. To investigate this issue, perforation intervals. Toinvestigate this issue, a method was developed to determine the existence ofhalite-cemented zones in wells for which only well logs are available. Thedistribution and dimensions of these zones were studied from a gelogicalviewpoint by looking at the deposition history and from an engineeringviewpoint by examining the production history with numerical well productionhistory with numerical well models. Numerical models also were used to studythe influence of the halite-cemented zones on salt deposition in the wells andon reservoir management. Field Description The Azal field is situated in the Marib AlJawf basin, which trends northwestacross the southwestern corner of the Arabian Peninsula. The basin ischaracterized by a Peninsula. The basin is characterized by a half-grabenstructure divided into several sub-basins, one of which contains the Azalfield. The dominant fault trends are parallel to the basin axis(northwest/southeast). The Azal field is bounded on the southwest by asouth-dipping fault and on the north and east by a dip enclosure. Thestratigraphic time setting of the Mesozoic formations is still subject todiscussion. The term "Alif formation," the main hydrocarbon-bearingformation, refers here to marine, deltaic, and fluviatile sequences below themain evaporite sequence (Main Salt) and above the turbidites (Upper Amran or Lam formation). Palynologic evidence indicates that the deposition of theentire Alif formation occurred during the Tithonian stage. Five geologicalunits can be recognized in the field (Fig. 2). The marine shales of the local Sean member contain turbidite sandstones. A delta shoreline facies forms thetransition zone between the marine shales and the overlying sandstones of the Alif member. The fluviatile and deltaic facies of the Alif member consist ofclean sandstones. The overlying transgressive shale member consists ofsandstones and shales and reflects the beginning of a major transgression. P. 518