Experience and Evaluation of Production Through High-Rate Gravel-Packed Oil Wells, Gullfaks Field, North Sea

Abstract

Summary This paper presents field data and performance analysis for high-rate gravel-packed oil wells in the Gullfaks field. More, than 2 years' worth of production history is available for the earliest gravel-packed wells. From extensive production testing and reservoir monitoring, the initial performance and performance history before water breakthrough was evaluated. An addendum reviewing later developments (1991–1993) is also included. Introduction The Gullfaks field, operated by Statoil A/S, is in the central part of the East Shetland basin in the northern North Sea, 175 km northwest of Bergen, Norway. Developed with three Condeep(TM) platforms (Gullfaks A, B, and C) the field started production in Dec. 1986. Of a total of 102 planned wells, 56 were drilled as of June 1991; six are subsea satellite wells. Estimated production life of the field is 20 years. The main drive mechanism is water injection. Predicted ultimate oil production from the field is 230 × 10(6) Sm3. The oil is located within three major sandstone units, the Brent Group, the Cook formation, and the Statfjord formation. The lower Brent delta sequence comprises the Broom, Rannoch, Etive, and Ness formations; the Upper Brent consists of the Tarbert formation. The reservoirs are overpressured, with an initial reservoir pressure of 31 MPa at datum depth (1850 m below mean sea level], and a temperature of 70 degrees C. The shallow, highly porous sands generally are poorly consolidated. The oil is undersaturated, with a saturation pressure of approximately 24 500 kPa, depending on depth and location. The oil viscosity is between 1.0 and 1.2 mPa.s at initial pressure conditions. Average horizontal formation permeability in gravel-packed wells varies from 250 to 10 000 md on the basis of transient well-test analysis. Porosity is high, generally between 35 and 40 porosity units. The Gullfaks field is divided into a number of fault blocks and is classified as a complex North Sea reservoir. During the first years of reservoir development, wells were naturally completed and selectively perforated in the relatively strong Rannoch formation. Considerable research was conducted to avoid sand production. After 2 years, development of the overlying weaker Ness and Tarbert formations began. Concurrently, a number of Rannoch wells were constrained by sand production initiated by water breakthrough. In these wells, water breakthrough severely reduced the maximum sand-free rate. Therefore, the need for sand control has increased with time. As a result, 2 years after production started, the completion strategy was changed to gravel packing. By June 1991, 14 platform oil producers were cased-hole gravel-packed. Earlier publications on high-rate gravel-packed oil wells discuss rates of up to 750 Sm3/d. The maximum oil rate produced through a single Gullfaks gravel pack is 5000 Sm3/d, limited only by calculated erosion velocity in the tubing. Gullfaks gravel-packed wells have been production-tested extensively, and laboratory work has been performed for better understanding of gravel-pack performance. This paper presents field data and its evaluation as of June 1991.

Production Enhancement From Sand Management Philosophy: A Case Study From Statfjord and Gullfaks

“…Even in cases where active sand control is chosen, solids production can not be completely prevented or ruled out. 7. Experience from both fields shows that wells produce sand prior to the onset of water breakthrough but that free water does increase sand transported to surface and can therefore result in further choking of wells.…”

“…Subsequent development of weaker overlying formations and water breakthrough in existing wells led to an increasing number of wells being constrained by sand production. As a result, a cased-hole gravel packing (IGP) completion strategy was adopted two years after production start-up 7 . Poor trends observed in well productivity due to scaling and fines mobilisation following water breakthrough and diminishing infill reserves has resulted in a return to natural completions on Gullfaks.…”

Scale Inhibitor Squeeze Treatment Strategy on Heidrun

“…The combination of low water cut, high draw down (over 100 bar in our internal gravel packed wells) and large GOR is especially critical, as observed previously in Gullfaks field. 3,4,5,6 To illustrate this the saturation ratio for CaCO 3 from the reservoir to the separator for different water cuts in Fangst fluids is shown in Fig. 4.…”

Use of Viscoelastic Surfactant-Based Diverting Agents for Acid Stimulation: Case Histories in GOM

“…Gravel packing has been extensively used as a standard sand-control treatment in the GOM for several decades. Before or after the placement of gravel with completion fluids, low-density brines, or linear gels, an acid treatment is usually pumped to remove the nearwellbore formation damage or high skins encountered in wells perforated overbalanced (Unneland and Waage 1996;Matherne and Hall 1996). In other cases, a clay-acid package is often pumped into the formation before the gravel pack to stabilize the residual clays.…”