Fracture characterization is a biggest challenge for the geoscientists insandstone and carbonate reservoirs. With the advancement of all technicalcapabilities in the acquisition of surface and subsurface Geological data, still it is extremely difficult to understand, characterize, and predict thedistribution of fractures in a field. Image logs can successfully be used tolocate and to provide directional trends of fractures near the wellbore.However, capturing all the fractures in one well and to predict their flowbehavior can still be a challenge. In this paper, a case study of a fracturedcarbonate reservoir will be presented. The field is currently producing about500 bbl of oil per day through fractures. Four wells have been drilled on thestructure to drain the oil reserves. Water flooding is being carried-out in thefield for the last 9 years for pressure maintenance and now 80% water is beingproduced. The reservoir has very low primary porosity and permeability, and theflow is through fractures only. On the basis of the three wells fracture data,a new well was drilled, located ideally at a structurally higher position, increstal area of the field. Image data showed abundance of fractures withdifferent orientation was seen in the well bore but the well didn't flow andthat led to its suspension. In this study, fracture data from image logs iscompared with outcrop analogs and seismic reflection and interpretation data.In this paper, limitation of the available information, importance ofunderstanding the stress regime, integration of G & G data and lessonlearned from the current evaluation of the fracture system and their impact ondevelopment of field in Potwar basin will be presented. Geological and Reservoir Overview Fimkaser oil field was undertaken as a case study for the understanding andcharacterization of the fracture. The field is located in the Himalayanforeland in North Pakistan and represents fault related anticline as shown inFigure-1. It was discovered in 1989 by Gulf Petroleum and later on handed overto the OGDCL. The producing formations are Chorgali and Sakessar limestone ofEocen age. These reservoirs are generally of low/none matrix porosity (1–3.5 %)and may be classified as type-1 of Nelson (1982), in which fractures provideessential porosity and permeability. In these types of carbonate reservoirs, secondary mouldic/vuggy and fracture porosity is important for storage ofhydrocarbons.

Exploration and exploitation of low permeability gas reservoirs in mature developed field is a challenge. It involves re-evaluation and characterization of reservoir properties of the producing zones. As the field understudy is mature and needs to understand the different pressure regimes and available gas potential in depleted and un-depleted zones before embarking on the exploitation of these low permeability reservoir. In past, development of low permeability reservoir was considered uneconomical due to low flow rate and longer pay out time. However, present gas prices and existence of infrastructure of surface facilities on field shifts the negative economics of low gas flow rates to positive direction. Consequently, any gas flow rates are deemed to be economical and a source of extra revenue. In this field, both carbonate and sandstone are producing reservoirs. Carbonates are of low permeability reservoir while sandstone is fragile and represent high permeability reservoir. Initially DSTs were conducted in both reservoirs while wells were completed in sand stone reservoir. Due to the fragile nature of the sandstone formation, gravel pack completion was used to avoid sand production. Initially, high permeability sand stone formation was produced. Volumetric and dynamic data was collected and analyzed to determine the undepleted part of the reservoir. The dynamic data indicated that low permeability reservoir has insignificant contribution and needs to be developed. Petrophysical properties of the carbonate reservoir were reviewed; new DST was conducted in the carbonate reservoir. During DST, longer build up was planned for reliable pressure data. After DST analysis, it was observed that it is low permeability reservoir with high skin value and need to be stimulated. Acid stimulation was carried out that have resulted in significant improvement and the well was put on production with sustainable production rate. In this paper, integrated approach of evaluation of geological and dynamic data that have resulted in successful recompletion of wells in low permeability carbonate reservoir will be discussed. The lesson learned from this successful evaluation and production of low permeability reservoir can be used for the exploitation of these reservoirs in the country.

Balanced drilling through naturally fractured reservoir and controlling loss for preventing reservoir damage and rehabilitation of normal production is a serious challenge in the Kohat-Potwar basin of Pakistan. The potential of hydrocarbons in these reservoir rocks has been masked by the overbalance drilling practices in this region. Due to overbalance drilling in fractured reservoirs and the use of heavy mud with barite blocks the fractures and that results in little or no flow during DST. The negative results of DSTs usually force the decision makers either to abandon the well or to re-test and establish the connectivity between the formation and the well bore.The well under study was drilled in fractured carbonate reservoir rock to a depth of more than 5000 meters in Kohat-Potwar basin to target Datta and Lockhart formations. During drilling, due to complexities, well could not reach the Datta formation. No wire line and image logs could be obtained in Lockhart formation due to slim hole. The last 5-7/8 inch hole of this well had to be drilled by using Oil Based Mud (OBM) to control well bore instability, the same mud was used in the reservoir sections. During drilling, losses were observed in the reservoir section. On the basis of drilling information, the well was directly completed in the Lockhart formation. After completion, well was allowed to flow but no hydrocarbon surfaced. As Lockhart formation is proven producer, and it became a challenge to evaluate the reservoir for its production potential and to find out the causes of no flow from the formation.After negative results of well test, all the data of G & G and mud logging was reviewed and detailed analysis of fractures network over the field were carried out to understand the well behavior. The data revealed that mud losses during drilling are indicative of fracture's presence in the tested zone(s) and fractures may have been attributed to an integrated approach and team work.