Ever since Shell introduced underbalanced drilling (UBD) in the offshore environment in 1997, Shell has been looking at ways to reduce the footprint for the equipment required for underbalanced operations. Much of this work involved reducing the size of the separator (volume) and modularising the UBD kit by stacking and containerising. As UBD takes a foothold in low-cost operating areas, Shell has actively looked at ways to not only reduce the footprint but also the unit operating cost driven mainly by the need for experienced expatriate personnel to man the equipment. Recent introduction of new or improved tools and equipment combined with growing recognition that the primary value driver for the application underbalanced drilling is dynamic reservoir characterization, creates a window of opportunity to move this initiative forward.
This paper presents the conceptual framework to replace large, manpower intensive surface equipment with a combination of down-hole and surface tools and equipment having the same functionality but improved efficiency, which enable a reduced footprint.
It illustrates how the functionality of the 4-phase separator can be replaced by combining inline phase separation, a gas buster and down-hole multiphase metering. Furthermore it describes how massive surface snubbing systems are being replaced with down-hole isolation systems and proposes replacing current intrusive and high maintenance BHA equipment with low-cost rotary steerable systems, non-intrusive MWD systems and the development of non-return valves (NRVs) that are compatible with through bore solutions to optimise data gathering while drilling. The paper postulates that every piece of equipment is driven by a functional need, has an impact on efficiency and therefore cost and may offer opportunity for enabling add-ons if the underbalanced drilling process is viewed from a different perspective.
Introduction
It has been pointed out many times and by as many folks, that underbalanced drilling is not a new technology. In its simplest form it is "making hole" with cable tool rigs. The next evolution was rotary air drilling, followed by air hammer drilling; underbalanced techniques designed primarily to improve rate of penetration in hard formations. Mist and foam drilling were technological tweaking to mitigate water influx to the well bore and to aid in hole cleaning while primarily drilling with an aerated fluid. In the mid to late 1980's, drilling high angle or horizontal boreholes to maximize exposure to reservoir rock, created the need for further enhancements but for a different purpose; to reduce formation damage caused by fluid invasion as a result of the longer exposure time required to drill the lateral sections. Today, underbalanced drilling has evolved where the primary well control function of the mud column used in conventional drilling, is replaced by a combination of flow and pressure control. Bottom-hole pressure and return well flow are continuously measured and controlled by means of respectively, pressure while drilling (PWD) measurements and a closed circulating system. Figure 1 illustrates the complete UBD system, which comprises of the drill pipe circulating system, a rotating control device (RCD), a UBD choke manifold, a separator and a flare stack or flare pit. In addition, non-return valves (NRV's) are installed in the BHA and drill string to prevent flow up the DP1.
As part and parcel of the evolution of this technology, a new term "Managed Pressure Drilling" (MPD) has taken root and like a tree, it has many branches.
The International Association of Drilling Contractors (IADC) defines Managed Pressure Drilling (MPD) 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 an attempt to reduce confusion, the underbalanced drilling industry represented by the IADC Underbalanced Operations and Managed Pressure Drilling Committee have also agreed to the following definitions for UBD and UBO.
Underbalanced Drilling (UBD) is "drilling under conditions where the pressure being exerted inside the wellbore from the drilling fluids is less than the pressure of the water, oil or gas in the formation being drilled".
