A seven well Big-Bore Underbalanced drilling and completion program was undertaken under challenging downhole conditions in a low pressure, high permeability carbonate reef reservoir in Indonesia. Underbalanced drilling technology was utilised as conventional drilling practices were no longer viable due to the extreme low pressure (1.1ppg e). UBD was selected as it would eliminate lost circulation and stuck pipe problems, reduce formation damage, eliminate the need for post drilling stimulation and give early analysis of reservoir behaviour and production rates. The resulting wells incorporated the largest tubingless gas completions in the world and were completed in an undamaged state, resulting in 29% higher than anticipated initial gas production rates. This paper will describe the Front End Engineering Design, Project Management, Risk Mitigation, Detailed Engineering & Design, Operational Results and Lessons Learned for this project. Introduction Located in a remote part of Indonesia, the field to be discussed was approximately twenty six years old at the time of the project, and the reservoir pressure had been falling rapidly from the original BHP of 7,100psis to a BHP of +/− 600psia at the time of the project. Conventional drilling techniques have proved unsuccessful for drilling new wells into the reservoir (the last attempt was in 1995) because the formation was unable to support the column of drilling fluid, leading to lost circulation and stuck pipe. In 1998, six wells were drilled underbalanced after an extensive pilot test, which utilised custom built equipment1. The ensuing studies and analysis pointed to an underbalanced Coiled Tubing Drilling (CTD) campaign. However, subsequent wellbore design, production demand and economics led to the conclusion that Big Bore (large diameter wellbore) wells drilled underbalanced with jointed pipe would be the most economical solution. This case study will provide details of how the project was progressed from the initial concept to use CTD up to the successful implementation of a seven well UBD program: A detailed Basis of Design (BOD) was developed to review the key drilling issues and make recommendations for how to best proceed with the project. Tender documents were issued, basic design was undertaken and contracts were signed. The detailed engineering & design phase was then conducted to refine the initial concept. This phase also included Quality, Health, Safety and Environment (QHSE). Seven wells were successfully drilled; the results of these operations and the lessons learned will be discussed later. Key Project Drivers The key project drivers were:Rapidly depleting high permeability and vugular limestone reservoir with ultra low reservoir pressure - static reservoir pressures on Cluster III ranged from 570 to 585 psia and in Cluster IV from 650 to 700psia. The reservoir pressure (1.1–1.4ppg e) was depleting rapidly at 0.3–0.4 psi/day (Figure 1). The speed of implementation was critical because with the rapid pressure depletion of the field it would soon not be possible to drill these wells underbalanced.Conventional drilling was no longer viable or economic due to massive losses with a mud system, mechanical and differential sticking, low ROP and formation damage (high skin) caused by overbalanced (conventional) drilling, which in turn reduces production rates. Drilling underbalanced would minimise/eliminate all the above conventional drilling problems and reservoir concerns.Reduce/eliminate the need for post drilling well stimulation.Provide early analysis of reservoir behaviour and production rates.

One of the many benefits of a managed pressure drilling (MPD) system is the reduction in the non productive time associated with kick and loss events. While such an approach has merit, a pressure determination system (PDS)1–6 has been developed to progress MPD from a reactive system to one which anticipates changing formation pore and fracture pressure regimes as the well depth increases. Ultimately the objective of the PDS is to prevent a recordable well control event from occurring over the duration of the drilling process. The PDS is deployed in conjunction with an MPD Pressure Control Valve (PCV), a rotating or non-rotating annular sealing device, and a flow metering sensor system. The PDS is based on the premise that a small ID PCV, positioned in parallel with a larger ID MPD PCV, oscillates with a programmed open-close cycling speed to generate a pressure pulse in the drilling returns annulus. The programmed PDS PCV thus produces the annular "pulse" with amplitude parameters specified by the operator within the PDS control system that oscillates the annular pressure within a predetermined narrow pressure band while keeping the overall average annular pressure constant. As the cyclic annular pressure changes occur, the models and algorithms within the PDS analyze the relationship between the return flow rate measured by the flow meter sensor and the surface PCV pressure to determine if either pore or fracture pressure margins have been breached. The PDS then readjusts the target bottom-hole pressure (BHP) using the MPD PCV such that the BHP continues to remain within the new drilling window. Please note that at no point is average BHP expected to fall out of drilling margins. Wellbore compressibility of fluids, solids, and gas, wellbore storage effects, and the efficacy of the pulse transmission are key factors to facilitate the analysis3. Since the PDS PCV is rapidly oscillating its orifice size, a degree of influx or loss is potentially expected to occur in the presence of changing pore or fractures downhole as drilling progresses further. The preset amplitude of the generated pulse either begins to increase beyond the fracture pressure (in the case of an unexpected decrease in the fracture pressure) or decrease below the pore pressure (in the case of an unexpected increase in the pore pressure). The result is that for a brief moment in time drilling fluid is lost to the formation or formation fluid enters the wellbore. What is critical to note is that the resultant loss or gain volumes are negligible and occur instantaneously with the associated peak amplitude of the pressure pulse as it dips below the pore pressure or above the fracture pressure. The flow meter sensor data analyzed by the alogirthms of the PDS detect these miniscule volumetric changes in the annulus and make adjustments before a recordable well control event can occur. Once the average BHP has neared any changes in the geo-margin limit detected and calculated by the pressure pulse analysis of the PDS, the MPD PCV can be manipulated to change the average BHP to continuously remain within the drilling window. Therefore, a recordable well control event is prevented. The PDS will proactively "ascertain the downhole pressure environment limits" as stated in the IADC definition of MPD. This paper will discuss the engineering concepts, practical implementation, and a preliminary field testing program for the PDS system.

No abstract

No abstract