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In excess of 100 horizontal wells have been drilled underbalanced in the Weyburn Unit in S.E. Saskatchewan. Initial underbalanced drilling (UBD) operations intermittently placed horizontal wellbores in an overbalanced condition due to large variations in bottomhole pressure while drilling. Pressure fluctuations acting at the sandface were found to have resulted in reduced productivity in many of these wells by forcing drilled fines into productive pore throats. In an effort to reduce formation damage, UBD procedures were modified to better maintain an underbalanced condition and decrease the magnitude of bottom-hole pressure surges occurring during drilling operations. Initial steps taken to optimize Weyburn UBD operations included the use of electromagnetic survey tools, the acquisition of bottomhole pressure data and adding on-site UBD technical support. This led to the implementation of basic operational practices such as minimizing survey and connection times, displacement of the drill string past the first float to nitrogen prior to making connections, shutting in the annulus during connections, and adjustment of circulation parameters to account for formation fluid influx, penetration rates, and wellbore length. Wells drilled using the adjusted UBD practices resulted in approximately 40% more production, on average, than those wells drilled using the initial UBD practices. Introduction During conventional drilling operations the hydrostatic pressure exerted by the column of drilling fluid is, by design, greater than the formation pressure of the zone being penetrated. This overbalanced pressure differential promotes both drilling mud filtrate and particle invasion into the target formation and can reduce well productivity. Fortunately, stimulation techniques can reach beyond damaged zones in most cased wellbore applications to provide access to undamaged reservoir. Horizontal well bores, however, are generally more susceptible to drilling induced damage than vertical wellbores due to their increased exposure time to drilling fluids and increased circulating pressure losses as horizontal length increases(1). Many horizontal well applications also involve underpressured or pressure depleted reservoirs, and are frequently completed open hole. Damage removal in these wells is often costly. Underbalanced drilling (UBD) has emerged as an effective technique to reduce weIIbore damage, particularly when drilling horizontally into underpressured formations. UBD refers to the drilling process involving downhole circulation systems designed with a hydrostatic pressure lower than the pressure of the reservoir section being penetrated. Properly designed, the process results in a continuous and controlled influx of formation fluids into the wellbore while drilling. Although both the magnitude of wellbore damage and the impact that damage has on well productivity are difficult to quantify in most horizontal wells, laboratory evaluations(2) have measured the effect that overbalanced, balanced, and underbalanced conditions can have on formation damage. These tests have demonstrated that underbalanced pressure conditions can significantly reduce the potential for formation damage both y mud solids blocking pore throats and by filtrate invasion. Field tests(3) have similarly identified benefits of UBD.

Controlled pressure drilling (CPD) is an emerging, multi-component drilling application that can improve drilling time and reduce previously recorded lost time on offset conventionally drilled wells-when applied appropriately. In addition, specific drilling techniques offered within CPD technology not only improve the time it takes to reach total depth (TD), but also have been proven to optimize reservoir section productivity and maximize economically recoverable reserves. CPD is composed of three main techniques: air drilling, underbalanced drilling, and managed pressure drilling. This paper provides an updated discussion of CPD techniques, the nonproductive time that it can address and screening methodology.A methodology to properly screen CPD techniques to reduce failure/misapplication and align objectives with expectations had been lacking. This paper addresses the latest refinements in an expert system developed to better understand and screen options for CPD operations. The Internet-based selection tool provides guidance by using key indicator questions, beginning with the primary objective(s) for the given wellbore section. If the specific CPD technique is known, the user can activate the online screening tool. The online screening tool considers a range of economic and technical parameters, which are applied to algorithms and logic rules, to provide a relative ranking for each CPD candidate. This paper will also address modifications that have been made to reduce cost related sensitivity when screening air and managed pressure drilling, which previously made the screening process less consistent. Sensitivity analysis can also be performed to determine the impact of key uncertainty parameters. Additional expert-user screening and selection can be incorporated upon the completion of the online CPD candidate selection process. Lastly, the paper will also include supporting case histories.

In 2005 a sour underbalanced horizontal gas well was designed and drilled in a pristine, environmentally sensitive region of southern Alberta, Canada. There were two primary drivers for the client. One was to remain under reservoir pressure throughout drilling and completion operations and the second driver was to safely recover 9.0% hydrogen sulfide (H2S) gas production and recycle the H2S contaminated drilling fluid. The well was not classified as critical sour because of its proximity to populated areas and a low well productivity of 0.015 E3m3/kPa (0.37 Mscf/psi); however, a cumulative flaring limit of 240 E3m3 (8.5 MMscf) was imposed by Alberta's Energy and Utility Board (EUB). Casing and hole sizes were optimized after confirming the well could be successfully drilled underbalanced to total depth (TD). Both well engineering and process engineering were integrated to provide the process and equipment to accommodate the sour well effluent while controlling the downhole multiphase flow and pressure environment. The injection gas for the underbalanced section was supplied via on-site compression from a sales quality gas supply line at a rate of up to 55 sm3/min (1,950 scf/m) at 10 000 kPa (1,450 psi). National Association of Corrosion Engineers (NACE) certified equipment was employed to create an enclosed gas and liquids separation system. The sour return gas was recompressed down an existing pipeline to a gathering station approximately 10 km (6.2 miles). To ensure the safety of the rig floor personnel, the drillstring was purged with an inert gas to the top float, and then bled off back to a low pressure separation system for drillstring connections. The well was successfully drilled underbalanced for 21 days, with 1.3% (8.5 E3M3)(0.3 MMscf) of the cummulative gas being flared. The use of recycled and recovered natural gas has economic benefits when compared to conventional disposable inert gas supplies, especially on a project basis. Introduction Conventional drilling of the underpressured, dolomitic limestone in the area created non-prodroductive time (NPT) associated with lost circulation and differentially stuck pipe. More importantly, the driver for this project was the induced reservoir damage caused by fluids and fines migration into the formation. To minimize formation damage, the well would need to be underbalanced drilled (UBD), thereby eliminating fluid loss, differential sticking, and the resulting fluid invasion that caused irrecoverable reservoir damage. The challenge of drilling of this field using underbalanced operations was that the reservoir not only contained methane, but also condensate and 9.0% H2S. The condensate and methane normally does not generate much concern for UBD other than an additional point of separation for the condensate prior to it reaching any atmospheric systems. The decision was made to employ recycling and recovery technology for both the drilling fluid and the lift gas to ensure the UBD operations could be completed without exceeding the EUB imposed flaring limited. Methane was used for the lift gas and was supplied from and recovered to a neighboring gas sales line at an average drillpipe injection rate of 52 sm3/min (1,830 scf/m) and 7,000 kPa (1,020 psi). Operational challenges, such as flush events causing over pressure shut downs on the suction side of the compressors, resulted in less than 1.3% of the cumulative net gas being flared on location. With sour underbalanced operations, there are many things to consider, from the drilling program to the emergency planning zone to the underbalanced surface equipment and results. The objective of this paper is to examine the pre-engineering, implementation, and the success of drilling a non-critical, Level 1 sour reservoir underblanced using recycling and recovery technologies and techniques.

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