Managed-Pressure Drilling; Techniques and Options for Improving Efficiency, Operability, and Well Safety in Subsea TTRD

Abstract

Subsea field developments are generally recognized as having lower recovery factors than fields developed by fixed installations. To increase the recovery factor from subsea developed reservoirs, new technologies that will reduce the costs of infill drilling and allow for more cost effective well interventions, must be developed. One potential technology is Through Tubing Rotary Drilling (TTRD). However, for the industry to perform extended reach TTRD from existing subsea producers using floating rigs, the way we manage pressure must be re-evaluated. TTRD combined with Managed Pressure Drilling (MPD) will be the key technologies needed to achieve the low cost, high performance drainage points. This paper describes several MPD methods that can be combined with TTRD and how these methods can be classified, evaluated and applied. Specific results from theoretical simulations will show how two different MPD methods can be used to drill longer departure drainage points than with conventional pressure control. Successful TTRD is believed to produce low cost drainage points for a fraction of the cost of a new subsea well. Introduction Some of the reasons behind the lower recovery factors from subsea developed reservoirs are: 1. Reduced accessibility to the well for interventions, repair and workover purposes 2. Lack of cost efficient well intervention tools and methods 3. High cost of new wells for infill drilling purpose 4. Escalating tangible costs and dayrates of Mobile Offshore Drilling Units (MODU) Effective well spacing and well placement in the producing reservoir is recognized as requirements for optimum reservoir drainage. The ability to access bypassed oil and gas reserves in mature fields has been gaining more and more attention in recent years. Mature fields have huge reserves that lie in multiple isolated pockets that would be uneconomic to produce using new wells. This may particularly be the case in subsea fields in deeper waters where the soaring dayrates for mobile offshore drilling units (MODU) will make the minimum economical reserve requirements hard to find. Through Tubing Drilling (TTD) is a method that eliminates the need for expensive conventional (new) wells or sidetracks. Avoiding drilling the “transport distance” down to the reservoir reduces the costs significantly. In addition, the re-use of the inplace completion equipment saves time for removing the old completion, time for running the new completion and CAPEX of the new completion. Coiled Tubing Drilling (CTD) has been the preferred and dominating TTD technique from fixed installations. However, when a drilling rig is available, the use of jointed pipe and rotary drilling operations has gradually become the more attractive option. The main advantage of using TTRD is the ability to rotate the drillpipe which improves hole cleaning, drilling mechanics, and ultimately increases the reach capability. Thus an obvious potential application of TTRD is infill drilling to access new reserves in subsea wells. TTRD in subsea fields faces several challenges. Many are associated with the narrow annulus between the production bore and the drillpipe, and to the variable formation pressures and lower fracture strengths in depleted formations. Another of the industry’s concerns of TTRD is the potential for wear/damage of the tubing and downhole safety equipment. Subsea TTRD operations are at the present in its infancy. Subsea TTRD has been performed in horizontal 7 in. monobore completions on the Norwegian continental shelf by Norsk Hydro on the Njord field and by Statoil on the Norne field. These operations have been conducted using a conventional drilling riser package, consisting of a 21 in. marine riser and a 18 3/4 in. subsea blowout preventer (BOP) package. Several new tools and procedures have been developed to protect key elements in the completion string and in the subsea christmas tree. However, this conventional riser and BOP set-up will significantly increase the challenges of incorporating MPD technologies with the TTRD concept. This paper will describe some of the options