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.

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.

Integrity of well structure is a vital factor to produce hydrocarbons fromsubsurface. It is important that well structure along with cement behind thecasings remains intact to get controlled and risk free production during lifeof a well. This objective is achieved by making every effort to ensure goodcement behind pipe and selecting suitable well tubulars honoring reservoirfluid composition and fluid flow rates. In case the criteria of well integrityis not met, flow of fluids behind casing and communication between tubularscreate problems and may lead to uncontrolled flow of well fluid creatingenvironmental and safety hazards. Bagla gas field is located within the Indus Basin with producing horizon inLower Goru Formation of Cretaceous age. This is a single well field, which wasdrilled by Phillips Petroleum Exploration Ltd during 1988. Later on OGDCLacquired this field. The field could not be developed due to its marginalreserves, remote location, non-availability of gas buyer in near vicinity ofthe field and non-attractive economics in case gas is sold to SSGCL due to highcost of pipeline quality gas to SSGCL gas transmission network. During April 2004 gas channeling to surface was observed in the nearby arealocated within a radius of 2–3 km from the Bagla well. This gas channeling wascausing environmental and safety hazards in the area. This paper discusses themeasures taken by OGDCL to diagnose the cause of problem and efforts made tocontrol the well to contain the environmental and safety hazards in thearea. Introduction Bagla is a single well gas condensate field located in Thatta Exploration Lease. The field is located about 150 km East of Karachi and 87 km South ofHyderabad. Nearest OGDCL well Nur # 01 is located at 3.5 km towardssouthwest.

During this period of energy crisis in Pakistan every effort is being made to produce every molecule of subsurface hydrocarbons. Particularly, the gas reservoirs which were not brought on production, due to low well deliverability or lack of required technology in the past are being explored and exploited. These include Tight, Low BTU, Sour and Acidic gas reservoirs. Such reservoirs pose specific problems related to drilling, production and development aspects.This paper depicts drilling and testing of a reservoir which is above sea level and its initial reservoir pressure is approximately 1000 psi below the normal hydrostatic pressure. It is one of the lowest pressure reservoirs of the world which has been drilled with successful flow of gas. Underbalance drilling technology was chosen to drill this challenging reservoir. Primary objective of under balance Drilling (UBD) was to establish reservoir potential by acquiring virgin reservoir characteristics.Historically, three wells have been drilled to test this reservoir. First two wells were drilled using conventional drilling methodology, both the wells experienced heavy mud loses during drilling and it was difficult to evaluate the production potential of this low pressure reservoir. Afterwards, pay zone of SML in third well X #02 was drilled and tested using Underbalance Drilling technique. This paper further describes the problems faced by the operator to drill first two wells in terms of mud losses and evaluation of production potential of low pressure reservoir of SML. In conclusion, it was a successful application which happened due to exceptional team work from all project parties. This application has opened new horizons of exploration and production of such reservoirs particularly in Baluchistan and generally in Pakistan.

The subject Gas Field is located in the Sulaiman Fold Belt (SFB) in Pakistan. A realistic 3D static model was constructed for the challenging multiple reservoirs in the Field which included both clastics and carbonates. Four main reservoir horizons were modeled. The steps involved in the Reservoir engineering analyses were: analyze PVT, well test, Static Pressure Data, and Core. The static pressure analysis helped define hydraulic compartmentalization in the field. WHFP measurements were not available in the desired accuracy and density. A surface network model was used with plant inlet pressure as the primary constraint in order to obtain the required information. Satellite based elevation information was used to establish an accurate model with respect to pressure drop due to liquid hold up in pipelines. The History Match in the field was performed on a Zone by Zone basis. In the absence of a 3D seismic cube, many of the faults in the field could not be interpreted, yet their presence was predicted by a closely matching Sand Box Model. This was an important clue which led to a useful approach regarding the location of simulation faults distributed in the entire field. An innovative approach was used in order to calibrate the size of sand lenses in one of the zones. The final step was the forecasting and development of Optimal Scenario using Economic analysis. Many scenarios were tested, and the optimal scenario was identified. Maximum use was made of existing wellbores through re-completion, and new drilling was minimized. Furthermore, the impact of increasing the currently low Gas Price was tested. It was concluded that doubling of the gas price of the field would increase the NPV 3 times delay abandonment by 6 years.