The southern part of Pakistan is known for its rich hydrocarbon potential. Most fields in the locality have matured, and pressure depletion has created problems in further field development. Located in northern Sindh, Qadirpur is one such field, which once accounted for 20 percent of Pakistan's total gas production. The highly fractured Sui Main Limestone (SML) formation is the primary reservoir in Qadirpur field, which has depleted over the years. Recent attempts to drill and produce from the SML formation were plagued with numerous problems including total circulation losses, extreme formation invasion damage, stuck pipe, and well-control events. Severe circulation losses not only led to excessive drilling time, but the heavy lost circulation material pumped into the wellbore invaded the formation and completely choked it off. The operator made multiple attempts to clean off the wellbore and kick off the well, but eventually no production was observed, ultimately leading to plugging and abandonment. The operator thus sought a solution that would enable drilling through the SML formation with maximum efficiency while minimizing formation damage during the drilling process. The operator desired to keep the SML formation in a "virgin" state from the beginning of drilling to the final production stage. The SML formation currently exhibits a low depleted pressure of about 3.9 ppg equivalent mud weight (EMW) and drilling through it requires a very low bottom hole pressure to mitigate circulation losses and subsequent invasion damage. Underbalanced drilling was thus considered for drilling through the SML formation in a proposed well, but a multitude of issues had to be addressed for successful execution of an underbalanced program. Some of the challenges in designing an underbalanced program on the subject well included maintaining underbalanced bottom hole pressures in the presence of high annular pressure losses, selection of appropriate directional drilling equipment for use with a lightweight multiphase fluid, maintaining efficient hole cleaning in the horizontal leg, designing an appropriate technique for tripping in and out of a live well, and finally achieving greater drilling performance to offset the cost incurred in underbalanced drilling. A nitrified foam system was selected to achieve an equivalent circulating density below 3.9 ppg. A special formulation of polymers was used in the base fluid for foaming to achieve enhanced cuttings carrying and suspension capacity. The directional drilling equipment was also customized to be used with a multiphase fluid medium. Lastly, a downhole isolation valve was deployed in the casing string to allow tripping of the drill string in a live well. By the application of underbalanced nitrified foam drilling, the operator was able to cut down drilling time by half and keeping the SML formation in a "virgin" state at all stages delivered an unprecedented production rate from the subject well.

Located in Sindh on the right bank of the Indus River, the Kandhkot Gas Field was discovered in 1959 and was estimated to contain up to 680 Bcf of gas reserves. The field contains three producing formations: Habib Rahi Limstone, Sui Upper Limestone, and Sui Main Limestone. The Sui Main Limestone formation has been depleted because of extensive production, and it currently exhibits a very low pressure of approximately 3.5 lb/gal (419 kg/m3) equivalent mud weight (EMW). An operator drilling with a conventional mud system encountered total losses and resorted to heavy lost-circulation material (LCM) pills and cement plugs to cure them. All bridging materials pumped in the hole to cure losses invaded the reservoir formation. As a result, the completed well required excessive cleanup operations to bring it into production. Therefore, the operator faced costs of loss curing materials, damage to the reservoir formation, and a substantial amount of time for drilling and cleanup operations. The operator sought a solution to not only reduce the costs associated with drilling this highly depleted reservoir formation, but also mitigate invasion damage that inevitably affects overall production volumes. Although underbalanced drilling had been used in similar situations, the operator considered the challenges of this well before execution. The horizontal well design contained a vertical section of about 1,640 ft (500 m). Challenges to account for while drilling the horizontal leg included designing appropriate directional drilling equipment for use with a multiphase fluid, cleaning the hole in the horizontal leg, maintaining underbalanced wellbore pressures in the presence of high annular pressure loss, and achieving drilling performance to offset the cost incurred by underbalanced drilling. For the given reservoir pressure of 3.5 lb/gal (419 kg/m3) EMW, a nitrified foam system was designed to achieve an equivalent circulating density (ECD) below 3.5 lb/gal (419 kg/m3) EMW. The foam system used a specially formulated polymer system to yield a high foam quality for suspension and cutting carrying capacity. The directional drilling equipment consisted of a special mud motor and extended range electromagnetic measurement-while-drilling (MWD) system to enable drilling with a multiphase fluid system. In addition, a downhole isolation valve deployed inside the casing string enabled isolating the live well while tripping. Using the custom underbalanced system, the operator drilled the well to total depth (TD) in just 3 days and achieved an ROP average of 49 ft/hr (15 m/hr). The paper outlines the planning and design of the underbalanced drilling package, the wellsite execution, and the achieved results.

As shallower reservoirs are driven to depletion the world over and the world energy demand keeps growing at a steady pace, operators explore for deeper horizons within current fields in hope of making significant discoveries. Deeper exploration in most fields entails significant risk, and a much higher cost per well. With deeper drilling depths comes tougher drilling challenges, mostly arising from higher pressures and higher temperatures at those depths. High-pressure, high temperature (HPHT) wells present numerous drilling risks, often including influxes while drilling into over-pressured formations, insufficient mud weight and bottomhole pressure control due to bottomhole density reduction with high temperatures, late kick detection due to low permeability formations, swabbing from the formation due to an insufficient trip margin, losses due to high equivalent circulating densities (ECD's), differential sticking, and stuck pipe following extended periods of well-control events. It is thus of paramount importance for the operator to minimize the associated risk, time, and cost on all HPHT wells. One such field in Pakistan where the target formation is a high-pressure shale in the Lower Basal Sand Reservoir, which is potentially a tight gas reservoir. This shale formation is known to be over-pressured, with a pore pressure of 18.6 ppg equivalent mud weight (EMW). The estimated fracture pressure of this formation is 19.2 ppg EMW, which results in a narrow drilling window. When an offset well in the field was drilled conventionally, it was plagued with severe well control issues, lost circulation, and stuck pipe events due to ineffective ECD management with a conventional mud system. The operator spent a total of 45 days to regain control of that well. The operator for the subject well therefore intended to deploy an automated managed pressure drilling (MPD) system to drill the target section with minimal nonproductive time (NPT). The MPD system was expected to facilitate drilling the section with minimal overbalance and compensate the required bottomhole pressure (BHP) with the application of backpressure. The automated MPD system would also account for mud density variations with a high bottomhole temperature (BHT) by executing an advanced well-hydraulics model in real time. Furthermore, the MPD system would provide early kick detection and reaction to well control events. The operator, in addition to drilling, intended to collect three whole cores while drilling with an MPD system. Through the application of an automated MPD system, the operator was able to reduce the NPT to practically zero, and successfully achieve target depth and collect the three desired cores. The paper discusses the planning, wellsite execution, results, and lessons learnt by the application of an automated MPD system in the subject field.

The Kirthar fold belt located in southern Pakistan, contains some of the largest gas reserves of the country. Operators when attempting to drill in the locality face a major hurdle in the surface hole sections. The surface section on most wells contain a group of three formations, Kirthar Limestone, Ghazij Shale, and Laki Limestone. Both the Limestone formations are highly fractured and exhibit total circulation losses, while the Shale formation is highly reactive and exhibits a swelling and sloughing behavior when drilled with a conventional water-based mud system. The operator on many occasions suffered stuck pipe incidents up to 3 times per well due to the massive circulation losses and shale instability problems. An air/foam system was initially used to eliminate the major problem of total lost circulation. The foam base fluid was formulated to contain a special blend of shale inhibitors to address the reactive nature of the shale formation. A special combination of polymers was also added to the base fluid to stabilize the foam in the presence of these shale inhibitors. The designed air/foam system was able to eliminate lost circulation completely and minimize the swelling and sloughing of the shale formation in a few wells drilled. While running the surface casing the operator observed high compression against the shale formation and in most instances the casing was set prematurely leaving a part of the vulnerable limestone formation exposed in the next section. As the next section required a much higher mud weight to drill, numerous cement plugs had to be peformed to bridge off the exposed limestone formation. The operator then further desired a solution that would enable them to drill without losses and allow them to land the surface casing to target depth. A drilling with casing system was first considered for this objective, but it was found to be incompatible with the air/foam system. The operator then finally decided to drill the section with an air/foam system and run the casing with a drilling with casing system, reaming through the troublesome shale formation. A series of wells were then drilled with an air/foam system, and the casing was run with a special drillshoe laced with pdc cutters. As expected high compression was observed against the shale formation. The casing was then connected to the top drive utilizing a specifically designed tool and reamed joint by joint to bottom. A conventional mud system was used while reaming the casing to bottom. The operator by the application of these two unique methods was able to drill and isolate the section in all future wells to target depth and achieve a cost and time saving of as much as 50 percent.

As oil and gas reserves mature the world over, operators are looking towards advanced methods of increasing the ultimate recovery from their ageing fields. An energy deficient country of Pakistan relies heavily on oil and gas imports. The country was once self sustaining in at least natural gas needs. A major portion of this gas was produced from the Field-X which was discovered in the 1950’s. The primary reservoir in Field-X is the YZ-Limestone reservoir which bears sour gas. Due to extensive production from the YZ-Limestone formation, the reservoir pressure has depleted to a mere 2.0 PPG in equivalent mud weight, and it being a naturally fractured limestone formation presents numerous drilling challenges. The operator has evaluated a potential higher pressured formation in the deeper horizons of sui field but that requires drilling through approximately 650-690 meters of the YZ-Limestone formation. This feat when attempted conventionally is plagued with numerous problems like, total lost circulation, differential sticking, influxes due to the loss of a sufficient hydrostatic head, and stuck pipe following well control events. To mitigate these challenges the operator, need an effective method to drill through this depleted formation without pumping heavy LCM pills, and multiple cement plugs across the massive cavernous thief zones in the YZ-Limestone formation which could have been detrimental to the production of nearby wells. Moreover, such remedies with specialized LCM’s and acid soluble plugs would have resulted in excessive material cost and non-productive time, which in some instances extended to a period of more than a month. To address the aforementioned challenges in drilling the YZ-Limestone formation, a multiphase managed pressure drilling system was suggested to drill the formation with minimal non-productive time and cost. Multiphase hydraulics were performed to assess appropriate pumping parameters for a near-balanced condition across the YZ-Limestone formation. A closed loop MPD equipment system was designed to help maintain near-balanced conditions in pumping and static (non-circulating) periods. The designed equipment system would also ensure that the risk of H2S exposure to the atmosphere was eliminated. The application of a closed loop nitrified mpd system on a recently drilled well proved to be highly successful and reduced the drilling time to just 28 hours by not only eliminating fluid lost circulation but by also delivering an extremely high rate of penetration of 39.2 m/hr. The successful and exemplary application of nitrified MPD has opened up a new horizon for the development of deeper prospects in the Field-X and similar neighboring fields. The paper outlines the design and execution of the closed loop nitrified MPD system.