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fax 01-972-952-9435. Novel ChemistryAs mentioned previously, the completions in these fields included dual and single laterals, approximately 1,000 to 3,000 ft in length. However, the dual laterals were planned without the utilization of a mechanical fluid-loss device. It was postulated that if re-entry of a lateral for post-completion cleanup purposes could be avoided, then risk, trouble-time, and costs would be reduced. It was further postulated that if a cleanup system could be pumped in the OH immediately after drilling to TD such that this procedure could be accomplished without the use of extra pumps, mixers, etc, then the planned operation for the upper lateral could proceed without the
fax 01-972-952-9435. Novel ChemistryAs mentioned previously, the completions in these fields included dual and single laterals, approximately 1,000 to 3,000 ft in length. However, the dual laterals were planned without the utilization of a mechanical fluid-loss device. It was postulated that if re-entry of a lateral for post-completion cleanup purposes could be avoided, then risk, trouble-time, and costs would be reduced. It was further postulated that if a cleanup system could be pumped in the OH immediately after drilling to TD such that this procedure could be accomplished without the use of extra pumps, mixers, etc, then the planned operation for the upper lateral could proceed without the
Shell is a major player in the Global deployment of Managed Pressure Drilling (MPD) technology to reduce drilling prob-lems, minimize formation damage and improve reservoir management techniques. The impetus is a recognition that most of the world's mature hydrocarbon reservoirs are in the lower end of the depletion cycle and an increasing number of horizontal wells are left with un-cemented completions. In addition, to access new untapped reserves, requires venturing into deeper water depths and the exploitation of deeper tight gas reservoirs, each with its own challenges. This paper describes the process involving rigorous pre-screening, planning, execution and feedback of learning de-veloped that enables fast implementation of underbalanced drilling (UBD) and integrated technologies into "Brown Field" areas and new business opportunities in "Green Field" devel-opment. This begs the question; what is Managed Pressure Drilling technology and what is the relationship with underbalanced drilling? The International Association of Drilling Contractors (IADC) defines Managed Pressure Drilling as "an adaptive drilling process used to precisely control the annular pressure profile throughout the well bore. The objectives are to establish the down-hole pressure environment limits and to manage the annular hydraulic pressure profile accordingly". In a sense managed pressure drilling, covers all drilling, since management of pressure is the goal of all drilling activities. However, there is general industry agreement that Managed Pressure Drilling is an umbrella term that refers to drilling activities conducted in a closed loop system. Conventional overbalanced drilling sits at one end of the Managed Pressure Drilling spectrum and underbalanced drilling at the other end. In conventional drilling, bottom hole pressure is managed by controlling the density of the drilling fluid to maintain well bore pressure profile above the pore-pressure throughout the well bore. Underbalanced drilling on the other hand, the well bore pressure profile is intentionally kept below the pore-pressure of all the exposed formations in the well bore. In be-tween these two extremes are various techniques used to con-trol the annular pressure profile and overcome constraints im-posed as a result of the equivalent circulating density of the mud system. Seen from Shell's perspective, Managed Pressure Drilling is a basket of drilling techniques that can be adapted to solve drilling related problems, reduce formation damage and or dynamically characterize production reservoirs (while drilling) to enable improved reservoir management. It is also worthy to note that not all Managed Pressure Drilling tech-niques require a closed loop system. Introduction Shell has deployed underbalanced drilling in oil and gas reser-voirs since the early 1990's. The introduction of horizontal drilling in the reservoir in the 1980's resulted in many new challenges. It created the need for a different approach to en-able drilling in depleted and fractured reservoirs. Shell initi-ated underbalanced drilling operations with foam1 in 1992 and in 1993 and 1994 the company conducted trials using multi-phase drilling fluids and the closed loop 4-phase separation system. In 1995, Shell successfully drilled the first horizontal underbalanced well using coiled tubing in Canada2. In 1997, Shell introduced underbalanced drilling in the offshore envi-ronment3. In the late 1990's, our Research and Development staff also developed and introduced expandable tubulars to the industry. A global approach to deploy these technologies was initiated in year 2000, when underbalanced drilling and ex-pandable tubulars were launched as two of four key technolo-gies (the others being 4D Seismic and Smart Wells) to be taken up within the Shell Group. Early on, the synergy be-tween underbalanced drilling and expandables was recognised as was the synergy between these two technologies and swel-lable elastomers and research and development work in these areas was supported.
This paper describes the application of underbalanced horizontal wells to develop some different tight gas reservoirs in the Netherlands section of the Southern North Sea. Underbalanced well design presented specific issues related to the nature of these reservoirs. The construction and production experience to date is evaluated and compared against similar wells drilled offshore UK. Also the benefits of the underbalanced horizontal well concept are reviewed. Introduction Most gas fields in the Southern North Sea produce from the Rotliegend sandstone formation that is of Permian age and deposited in both aeolian and fluvial setting. The Rotliegend reservoirs are characterized by a large variability. The average porosity varies between 8% and 18% with peak porosities ranging from 22–26%, while the average permeability varies between 0.01 mD and 100 mD. In parallel the compressive strength of the formation material shows drastic variations as well i.e. between 10 bar and 1000 bar. A significant portion of the Rotliegend reservoirs are characterised as tight gas, defined in this paper as having an average in-situ air permeability of less than 1 mD. Up to the 90's the preferred method of developing these types of tight gas reservoirs was through massive hydraulic fraccing. During the 90's the emphasis shifted to horizontal drilling. This proved successful in tight gas fields in the UK sector of the Southern North Sea where the (sub)horizontal drainholes connected productive natural fracture networks. In the late 90's underbalanced drilling (UBD) was introduced, in first instance to avoid the frequent drilling problems associated with total losses into these natural fractures. However, significant productivity gains were also observed and this became a key driver to apply the same UBD technology in tight gas fields in the Dutch sector of the Southern North Sea. This paper describes the experience to date offshore The Netherlands. Setting the Scene The average permeability cutoff used to define tight gas in the Southern North Sea is 1 mD or 10 times higher than the tight gas cutoff of 0.1 mD used onshore USA. The difference is due to the much higher offshore Europe well cost, which forces field developments to aim for a one order of magnitude or more higher well capacity and raised ultimate recovery to generate economic returns. Typically the initial well capacity offshore Europe should be of the order of 1*106 m3/d (compared to 0.1*106 m3/d onshore USA) while the ultimate recovery offshore Europe should be of the order of 1*109 m3 (compared to 0.1*109 m3 onshore USA). Hence the challenge has been to boost the well capacity and increase ultimate recovery of these tight gas wells to increase their economic attractiveness for development in the Southern North Sea setting. UK Tight Gas Experience In the 80's hydraulic fracturing of deviated wells was the method of choice for developing tight gas reservoirs in the Southern North Sea (Ref. 1). Although sound in principle, in practice problems were experienced caused either by poor cleanup due to fluid incompatibility, proppant back production causing fill and erosion of surface facilities or early water breakthrough due to fracturing into the water leg. In the 90's horizontal drilling became common practice as new drilling technologies developed and proved to be very successful in several UK Southern North Sea tight gas fields. Table 1 compares the productivity index (PI) achieved by hydraulic fracturing and horizontal drilling in 3 UK fields and shows 2–8 times higher productivity from horizontal wells. The success of the horizontal wells largely originates from intersecting productive natural fracture systems.
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