This paper describes ongoing efforts to improve the performance of Nimr wells by identifying and shutting off thief zones and summarizes results to date. The presence of a combination of unstable displacement, strong aquifer drive, complex geological settings and non-matrix permeability events has resulted in "scatter-shot" new well performance with progressively higher average initial water cuts. Although not a traditional fractured system, Underbalanced Drilling (UBD) data have shown that rogue fractures and bypass zones play a key role in the movement of water through Nimr reservoirs. The concept is to use UBD to identify "non-matrix" water bearing features (rogue fractures, bypass/thief zones), and shut them off using different shut-off techniques, including a new selective shut-off completion technology called EZIP. It is expected that this will lead to improved water cut behavior, and higher oil production rates and ultimate recoveries from new wells. A sector model of the reservoir was used to evaluate the impact of the different fracture shut-off techniques. Using the unstructured grid capability of the simulator, rogue fractures intersecting the wellbore are introduced at arbitrary locations, and the impact of shutting them off either along fracture length or along wellbore face is investigated. The simulator model provided a means to quickly study several sensitivities such as dimensions, number and orientation of fractures, initial water saturation, reservoir pressure, etc. The results of the model showed that properly designed fracture shut-off methods could delay water cut by as much as 600 days. Equally importantly, the model showed that shutting the fractures along their length (by cement or gel squeezes, for instance) has almost no beneficial impact on the water cut behavior of the wells during production. UBD was critical to the development and implementation of the shut-off technology. UBD was used to characterize not only the bypass features, but also their effectiveness and water production potential. Indeed, the authors argue that UBD is the only approach whereby static and dynamic characterization can be achieved while drilling. Based on interpretive analysis of data gathered during UBD, together with petrophysical data, non matrix features and their potential were identified, and used to decide upon the most effective shut-off strategy. The concepts were tested in a number of wells where bypass features producing water were identified. The first well used abandonment plugs to shut off the whole lower section of the well, while the others have used EZIP technology to shut off the features. Results from the tests have been very encouraging, and confirm the key conclusions of the dynamic modeling. Based on the study and well results, the Nimr Asset team is embarking upon a long term campaign of using UBD to characterize the reservoir and optimally complete the wells to improve water cut behavior in wells. The paper discusses the modeling, the reservoir characterization enabled by UBD, as well as the results from completed wells. The authors conclude that a combination of fit-for-purpose reservoir modeling, UBD enabled reservoir characterization and appropriate shut-off technologies can lead to significant value creation in fields like Nimr.
In May 2002, Petroleum Development Oman (PDO) embarked on a ten well, underbalanced drilling (UBD) trial campaign in the Nimr field using crude oil as the drilling fluid and membrane generated nitrogen as the lift gas. UBD was proposed as a productivity improvement technique for the Nimr field following a low risk/high reward analysis. The Nimr field is a complex of six fields. UBD was implemented in the Nimr A field consisting of two reservoirs: the Amin and Al Khlata, which are generally high permeability (±1Darcy) sandstone reservoirs containing medium gravity (21°API) viscous (300–500 cP) crude. Horizontal wells are generally completed with a wire-wrap screen (WWS) across the reservoir section, due to sand production history in some wells, and are produced via artificial lift methods, primarily beam pump. Even though the predominant factor affecting net oil rate performance was the rate and behavior of water cut development it was suspected that drilling-induced skin, combined with mechanical skin from the completion, was a contributing factor to recent poor results from the horizontal wells. The paper will demonstrate the value of a multi-well campaign to avoid eliminating a good candidate reservoir due to inconclusive start-up results associated introducing a new technology. It will describe some of these early start-up challenges, the equipment modifications and changes to operating procedures that have resulted in the uptake of this game-changing technology in the Nimr field. Additionally, it will emphasize the potential value of well inflow and reservoir characterization data gathered during UBD operations. This data indicated significant opportunities to improve well performance and increase ultimate recovery resulting in a potential value far exceeding those originally envisaged prior to initiating the UBD trial. Introduction In early 2000 SIEP (Shell International Exploration and Production B.V.) identified UBD as one of four key technologies to be taken up within the Shell Group on a global basis. Global Implementation Teams were formed to assist Group Operating Companies to prepare implementation plans that included screening exercises, candidate selection, business case development, and execution. Several of PDO's assets were considered good candidates for UBD and following a ranking exercise; two fields in Nimr and Saih Rawl were selected. A Low Risk/High Reward strategy1 resulted in a ten well UBD campaign in the Nimr A field (Figure 4). The objective of the campaign (similar to that in the Saih Rawl campaign being executed concurrently2) was to quantify the value of UBD for PDO. Nimr Project - Scene Setting Nimr is actually a complex of six fields. UBD was implemented in the Nimr A field which is a large "turtle-back" structure primarily composed of two reservoirs beneath the Nahr Umr shale cap rock:the Haima Amin (Cambrian, Aeolian dune) which is the primary reservoir; andAl Khlata (Permian, glacial) which is eroded into the Amin and is present along the faulted flanks of the field. The Amin and Al Khlata both have reservoir permeability of >1 D in the better quality sections. The Amin contains small cement streaks within the section, which may cause local baffling. The Amin also has 10–30+ m thick sections of weathered zone, which is of lower quality, permeability and oil saturation than the "good" Amin. The Al Khlata has been shown to have a high degree of lateral variability but is generally good quality over most of Nimr A field. (Figure 2 and Figure 3). The crest of the structure is at about 680 m SS and the original OWC is at 747 m SS. Oil density is 0.93 sg, viscosity is 300–500 cP at 50 °C reservoir temperature. Aquifer support is moderate to strong bottom water drive. The initial pressure gradient was 10.2 kPa/m and the current reservoir pressure gradients range from 6.5 to 9.5 kPa/m. The oil produces at a very low GOR (<1m3/m3) with intermittent traces of H2S.
The value of underbalanced drilling (UBD) and completion technology lies in several distinct areas. These include improvement in drilling performance by, for example, reducing losses or differential sticking, or increasing rate of penetration or even depletion of troublesome high-pressure zonesi. UBD is also used to improve production rates and increase ultimate recovery. In these cases, UBD has been applied to avoid formation impairment and thus to enhance well productivity. Recently, some operators have recognized that value can also be created through gathering and interpreting reservoir data while drilling underbalanced. This information is used to improve reservoir knowledge and consequently enhance reservoir management. In low-cost drilling environments, such as land operations in the Middle East and USA, UBD drilling-enabling savings are often marginal and the cost of UBD operations becomes a blocker for wider implementation. The extra cost of UBD acts to discourage a company from fully evaluating the technology's capabilitiesii, even following technically successful trials. In addition, historically the Well Engineering community has championed UBD programs often with scant involvement from the subsurface Petroleum Engineering (PE) disciplines. Petroleum Development Oman (PDO) operates in such a low-cost drilling environment. PDO commenced UBD campaigns in 2002 with the primary goal of enhancing production and has succeeded in sustaining UBD operations by making significant progress both in reducing costs and increasing the value of information derived from UBD to the Petroleum Engineering organization. This has created the opportunity to more fully evaluate the technical and economic benefits that UBD can deliver and the possibility to deliver cost-neutral UBD wells. This paper describes some of the challenges faced when initially introducing UBD and the step-by-step approach followed by PDO in Oman and Shell Exploration & Production Co. (SEPCo) in the USA to widen its implementation to other fields. The paper underlines the need for integrated well engineering and petroleum engineering teams in UBD and confirms the pivotal role of sub-surface engineers in interpreting and using the additional and complementary reservoir data that becomes available during UBD. The paper also analyses more closely the actual cost of UBD and provides a balanced perspective on the longer-term UBD value equation. Introduction Over the past 20 years in Exploration & Production, much of the business focus has been on value creation through unit cost reduction, i.e., spending less to produce incremental oil. This was driven primarily by the view that oil supply was, and would remain plentiful and that the oil price would not keep pace with general inflation. As oil prices proceeded to fluctuate between levels that allowed decent economic returns to near economic breakeven points for operators, the largest economic lever available to operators was constantly in flux, i.e., drilling budgets. In this business environment UBD projects, which have a relatively large initialization cost and add significantly to operating day rates, have been difficult to sustain through the budget cycles. Consequently, implementation of UBD has experienced growth and contraction concurrent with the rig count and continuous operation has been limited to North America and the southern North Sea, where it has been clearly demonstrated to deliver more value through lower costs and/or production enhancement over conventional drilling and completion techniques and for the past two and a half years in the Middle Eastiii,iv. This paper focuses on the ongoing efforts to establish UBD as the preferred reservoir exploitation method in PDO. These efforts are concentrated on highlighting, not only the usual production benefits associated with UBD, but also the cost reductions that will help PDO achieve its goal of cost neutrality of UBD when compared to conventional operations. Equally important is the value of other less obvious benefits, in the form of reservoir learning enabled through the analysis of UBD derived reservoir characterization (RC) data. In fact the longer term value of the RC data, properly applied, is likely to far exceed the value of cost savings.
To meet future global energy demand, access to deeper and harder to get at hydrocarbon reservoirs requires innovative and cost effective technical solutions. Managed pressure drilling (MPD) is one such solution that can best be described as an adaptive drilling process engineered to safely address exploitation challenges related to reservoir uncertainty issues and cost. MPD is a catchphrase for a whole suite of techniques (including underbalanced drilling UBD) that enhance operational safety, reduce costs, improve reservoir performance and ultimately increase asset value and profitability. MPD delivers economical solutions across the spectrum of drilling operations; managing top hole losses and well control problems in vugular carbonates, managing kick-loss challenges in narrow pore pressure/fracture gradient situations, improving production performance in fractured carbonate oil reservoirs, improving rate of penetration and production performance in tight-hard-rock gas reservoirs and enabling real-time dynamic reservoir characterisation and superior drilling and completion decision making. MPD in general relies upon a closed or semi-closed circulating system whereby flow and pressure in the well bore can be precisely controlled, thus enabling a safer, sustainable operation in higher risk environments. The Shell Group has to date deployed MPD overbalanced and MPD underbalanced in over 415 tight gas wells globally. Specifically, Shell has used UBD for dynamic reservoir characterization (RC) on tight gas wells since 2005. The knowledge and insight gained was key to the decision in June 2008 by the global exploration and drilling management teams for the strategic deployment of UBD for RC on all exploration and appraisal tight gas wells. Tight gas exploration plays typically have the following primary objectives; identify if the reservoir is gas charged, identify if the gas is mobile within the reservoir, identify the flow mechanism and flow units and identify the producible hydrocarbons associated with them. While the gas charge issue can often be determined from static logs and core, the remaining objectives deal with dynamic gas mobility uncertainty. The ability to characterize the reservoir with UBD constitutes a new tool to collect data and reduce project risk for the remaining dynamic gas behavior objectives. Since this decision was taken, UBD-RC has been, or is being deployed on 10 projects in 7 different countries. Shell considers integration of UBD deliverables into the exploration play planning and sub-surface team decision making critical and an area where the operating company must take the lead. This has resulted in optimized equipment specific to tight gas plays to reduce contract services cost and the use of geo-mechanical analyses to validate UBD feasibility and to enhance borehole stability management in a UBD environment. Production from intervals not previously considered productive has been observed and some insight into the behaviour of dual porosity systems has been acquired. This paper outlines the rationale for Shell deploying the technology for dynamic RC and the lessons learned in applications to date. This paper contains examples from projects in Europe, North America, Middle East and Africa. Examples from North America include projects in mature tight-gas fields to wildcat exploration wells.
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