TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractRasGas Company Ltd Drilling Task Force has instituted a new work process from ExxonMobil Development Company. This process is designed to optimize rate of penetration (ROP) in every foot of hole drilled. ROP limiters are systematically identified and eliminated during the drilling design, as well as during real-time well site operations.A central element in this work process is the real-time display and analysis of Mechanical Specific Energy (MSE). MSE is the calculated work that is being performed to destroy a given volume of rock. When a bit is operating at its peak efficiency, the ratio of energy to rock volume will remain relatively constant. This relationship is used operationally by observing whether the MSE changes while adjusting drilling parameters such as weight on bit (WOB) or rotary speed (RPM). If it remains constant while increasing WOB, the bit is assumed to still be efficient. If the MSE ratio increases significantly, either while drilling or while adjusting parameters, the bit has foundered. The driller then determines the most likely cause of founder and adjusts parameters accordingly. Adjustments continue to be made until the MSE value is minimized. The process of adjusting parameters is referred to as MSE testing. Recorded results of the driller's MSE tests are also used in post-analysis to guide redesign of the drilling system when ROP limiters are beyond the driller's control. In the North Field operations, downhole vibrations data were coupled with the MSE curves to further enhance the interpretation of the cause of founder and to manage drilling parameters.This paper presents examples from Qatar operations that demonstrate the manner in which MSE and vibrational data were used at the rig site to make operational decisions, and in post-drill analysis to redesign the system. Performance data is also provided to show the impact on operations. Performance improvements have varied by hole size, but range from 60 -380%.

In March 2005, the operator implemented a rate of penetration (ROP) management process in Qatar's North Field. IPTC Paper 10706-PP describes the general principles behind the new work process and highlights its introduction to the operator. The ROP management process uses real time, customized surveillance technology to continuously maximize both drill bit cutter efficiency and transmission of energy from rig floor to the bit. This paper focuses on specific changes in drilling practices and their translation into substantive program acceleration and capital savings. To date, the development program has been accelerated by one year and USD 54 million has been saved while drilling 470,000 ft of hole. The process has proven to be a highly effective solution for management of drilling efficiency in a major development drilling campaign. As an indicator of program scope and effectiveness, over 440 personnel have been trained in mechanical specific energy (MSE) analysis, and 50 new field drilling records have been set by the nine rigs involved in the program. Importantly, the process has been implemented with one of the best safety records in industry (TRIR or total recordable incident rate of 0.11 per 200,000 man-hours as of September 06). Introduction The Qatar operator currently conducts drilling operations with nine rigs at eight platform locations across six very large producing blocks, each typically a 12 × 12 km concession. The majority of wells are drilled in batch mode by section, an approach adopted in the late 1990's when development drilling began. Over the years, the operator gained extensive field knowledge and developed many efficient operating practices. Thus, the introduction of any new ROP management process would be scrutinized and assessed against solid baseline operating performance. At the same time, 2005–2006 has been a period of transition, as the company increased rig count from three to nine rigs. The addition of many new support personnel in a multi-cultural working environment presented a significant challenge in terms of maintaining or improving drilling efficiency. This was coupled with high service personnel turnover, which continues to pose operational challenges. In the face of these challenges, it was decided to pilot test the ROP management process, with its first application in such an extensive carbonate field environment. The focus of the program was fourfold:conduct extensive, customized field training in MSE analysis;implement a standard surveillance program;introduce new practices in a phased manner concurrent with measurement of energy efficiency; andsystematically communicate learning across all rigs using MSE curves as the basis for discussion. This paper discusses summary learnings and offers technical examples of how MSE is being used to achieve consistently better drilling results. Specifically, the role of lithology, the general approach used to manage efficiency, ROP limiters encountered, and BHA optimizations will be discussed. While learning specific to the North Field is presented, the information is applicable to many drilling areas. Qatar North Field Background Figure 1 shows the location of the North Field offshore Qatar. Figure 2 shows both stratigraphy and typical wellbore configuration. Aside from one vertical data well at each platform location, all wells drilled by the operator are 55–65° S-shaped directional wells with an abbreviated drop section into the Khuff reservoir. Although platforms are kilometers apart, many wells are quite similar in terms of lithology, hole size, casing configuration, and casing setting points. An important reference data set when discussing MSE is rock unconfined compressive strength (UCS), presented in Figure 3. The MSE recorded while drilling should vary as the bit traverses rock of various strengths. However, it should only vary by the amount of change in rock strength. In field operations, a baseline MSE is established and any increase above this that exceeds the change in rock strength is likely to be an indication of bit dysfunction. The trending nature of MSE surveillance is described in IPTC Paper 10706-PP. It should be noted the efficiency gains described in this paper are only those directly attributable to the ROP management process in 17–1/2″, 12–1/4″, and 8–1/2″ sections.

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An extensive drilling program is ongoing for the development of Qatar's North Field. This massive offshore carbonate field contains natural gas produced from the thick Khuff reservoir at approximately 10,000 ft true vertical depth (TVD). Capable of flow of over 200 MMScf/D each, these complex wells pose considerable challenges to both drilling and completion operations. One of the most significant challenges is over-pressured water flows which are encountered randomly within the primarily shale- and clay-stone section above the production zone. The major risks associated with these high-pressure flows include the masking of gas kicks within the massive sour gas reservoir below it; reduction of hydrostatic head during underbalanced operations and trips; added complexity to any simultaneous lost returns events; obtaining the required open-hole logs; and adequately cementing the production liner. During the ongoing program, the development of an advanced engineering and operational response to these persistent water flows has dramatically reduced their time and cost impacts without compromising safety or well objectives. Acquiring quality data to determine the characteristics of the "water flow reservoir" is the first step in effectively managing each well control event. Shut-in data is collected first to verify the influx is due to a water flow interval. If it is verified the influx is due to a water flow, inflow testing is done to assess the deliverability characteristics of the water-flow reservoir. Empirical equations derived from inflow test data are input into a model to calculate how the pressure exerted by the fluid column at the top of the gas reservoir will decrease over time given a specified mud density. The model assumes no wellbore circulation, as its primary purpose it to predict how long the fluid column pressure will remain overbalanced to the gas reservoir pressure during trips or other periods when the well is not circulated. Bottomhole pressure (BHP) vs. Time modeling is also used to determine what mud density will be required to be able to drill, log, and case the production-hole interval. If it is too high, the flow zone is then depleted until subsequent inflow data and modeling indicate the well can be finished with a resonable mud weight (MW). Weighting up to kill the zone or depleting the zone to a zero flow state is not practical. This paper discusses data collection, modeling, unique well-control risk-management practices, and operational procedures developed for effective wellbore construction in the presence of a persistent water flow. Introduction Fig. 1 shows the location of the Qatar North Field.1 Fig. 2 shows the field stratigraphy and a typical optimized big bore (OBB) wellbore configuration.2 The majority of the wells are similar in lithology, hole size, casing configuration, and casing setting points. They differ primarily in inclination and throw. Approximately 10 percent of the wells are vertical, with the remainder varying from 55 to 660 inclination to approx-imately 10,000 ft TVD.3 To date, over one million ft of hole has been drilled by the operator in this field. Previously published work addresses the drilling and completion of these complex wells.1 The operator's Drilling and Completion Department has experienced 12 production-hole water flow events during its ongoing Qatar North Field development drilling program. Eight of these were low-flow-rate (< 2 bbl/hr) events, which were more operational nuisances than barriers to operations. Four of the events had initial flow rates from 15 to 60 bbl/hr and posed significant challenges to continuing operations. The water flow zones are encountered in the Sudair formation, which is above the Khuff gas producing horizons. The Sudair primarily consists of shale and claystone with interbedded stringers of dolomite and anhydrite. Internal geological studies have been unable to predict where the flows are likely to occur; however, they have been able to correlate the flows to a 5- to 10-ft thick dolomite stringer. The permeability of this stringer varies over the extent of the field. The high flow-rate stringers have pore pressures of up to 16 ppg equivalent mud weight (EMW) and produce brine water. See Table 1 for brine characteristics.

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