Accessing Deep Reservoirs by Drilling Severely Depleted Formations

Abstract

In many maturing prospect around the world, operators are facing the challenge of having to drill through highly pressure-depleted formations in order to access lower-lying hydrocarbon-bearing zones. New technologies such as expandable casing are now becoming available to allow for extensions to conventional well designs in order to deal with depletion. However, before one can case off depleted formations, one first has to successfully drill them. This paper highlights key aspects in the planning and execution of the Ursa A-11 well, which was drilled through a 5500 psi depleted sand to a deeper horizon. Drilling complications included risks of excessive mud loss, internal blowout and differential sticking on the depleted sand. Moreover, fracturing of the depleted sand carried the risk of jeopardizing production at a nearby horizontal well. Key factors in the successful drilling of the Ursa A-11 well included special drilling fluid design, rock mechanics study, pro-active use of borehole strengthening technology, integration of supplier and operator expertise, and excellent communication between all parties involved. Introduction Producing a prospect's reservoirs "from the bottom up" may not always be feasible. Development economics often dictate that higher-reserves or better-quality reservoirs must be produced first before deeper-lying horizons can be accessed. In many maturing prospects operators are challenged to drill through zones that are severely depleted from past or ongoing production in order to unlock these deeper reservoirs. This situation applies to the deepwater prospect Ursa in the Gulf of Mexico (GOM). The main reservoir at Ursa is the Yellow sand, which is currently being depleted by three high-rate producing wells. Pore-pressures in the Yellow sand have typically dropped by 5000 - 6000 psi since production commenced in 1998. Production has not only reduced the pore-pressure but has also lowered the minimum horizontal stress in the Yellow sand (see Fig. 1). Such conditions greatly complicate accessing the Sub-Yellow reservoir, an untapped hydrocarbon-bearing zone at virgin pressure situated just below the Yellow sand. Significant challenges surfaced while planning the Ursa A-11 Sub-Yellow producer, for which the casing program is given in Fig. 2:The high GOM cost environment dictated the need for a high rate completion from a small Ursa template slot. Marginal economics on the Sub Yellow sand precluded any other development concepts (e.g. separate subsea well, use of a large Ursa slot etc.).Drilling risks included the possibility of an underground blowout from virgin-pressured sands above and below the Yellow sand (i.e. pore-pressures of adjacent sands are higher than the reduced fracture gradient / minimum horizontal stress in the depleted Yellow sand, see Fig. 1). Also, there was a high risk of differential sticking and associated loss of hole while drilling the Yellow sand at high overbalance (5500 psi).The optimum bottom-hole location for the Ursa A-11 well placed it in very close proximity (˜ 400 ft) to the high-rate Yellow horizontal producer Ursa A-6 (see Fig. 3 for a subsurface projection of the A-11 and A-6 wells). This introduced the significant risk of fracturing the A-11 well at the depth of the Yellow formation into the direction of the A-6 well. Propagation of drilling mud from A-11 to A-6 could result in impairment of the A-6 completion and thus compromise further production from A-6. To gain a proper perspective of the proximity of the A-11 and A-6 wells, it was estimated that hydrocarbons would be flowing by the A-11 well at an amazing rate of 2 ft/day due to ongoing production at the A-6 well. URSA A-11 Well Planning Significant effort went into the planning of the Ursa A-11 well to address the challenges associated with developing the Sub-Yellow sand. Planning was tackled by an integrated project team that included the Ursa prospect development team, drilling engineers and drilling fluids & cement team, R&D experts, and resources from various suppliers. Specific planning elements are discussed in detail below.

Near-wellbore Alteration and Formation Stress Parameters using Borehole Sonic Data

“…Near-wellbore alteration can be caused by several sources, such as borehole stress concentrations, drilling mud pressure, plastic yielding of the rock prior to breakouts, shale swelling, drilling-induced fractures, and invasion of monomer and resin materials in synthetic drilling muds used to strengthen weak formations. 1,[4][5][6] Estimation of the magnitude and radial extent of mechanical alteration helps in an optimal design of perforation tunnel length for improved flow rate in the presence of near-wellbore permeability impairment.…”

“…In addition, drilling through highly pressure depleted reservoirs raises considerable risks of excessive mud loss, internal blowout and differential sticking. 1 Drilling through such depleted sands was accomplished in the Ursa field in the GOM using water-based mud with monomer and resin materials that exhibit larger fracture propagation pressure than that of oil-based mud -even though the fracture opening pressures are similar for both the water-based and oil-based mud.…”

Setting Free the Bear: The Challenges and Lessons of the Ursa A-10 Deepwater Extended-Reach Well

“…1). Drilling such wells is a nontrivial exercise with special challenges (van Oort et al 2003). These include the following:…”

Managing Suspension Characteristics of Lost-Circulation Materials in a Drilling Fluid

“…Additional relevant literature within this area includes work by Van Oort et al (2003) and Dupriest (2005). In a laboratory setup, fractures are simulated in the drilling fluid lab using particle plugging apparatus (PPA) with either aloxite discs or metallic constant area slots.…”