Troll Horizontal Well Tests Demonstrate Large Production Potential From Thin Oil Zones
- Seines Knut (Norsk Hydro A/S) | S.C. Lien (Norsk Hydro A/S) | B.T. Haug (Norsk Hydro A/S)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- May 1994
- Document Type
- Journal Paper
- 133 - 139
- 1994. Society of Petroleum Engineers
- 2.4.3 Sand/Solids Control, 5.1.1 Exploration, Development, Structural Geology, 5.2.1 Phase Behavior and PVT Measurements, 5.6.4 Drillstem/Well Testing, 1.6 Drilling Operations, 5.3.4 Reduction of Residual Oil Saturation, 1.2.3 Rock properties, 2 Well Completion, 4.6 Natural Gas, 5.5.8 History Matching, 1.14 Casing and Cementing, 3 Production and Well Operations, 5.5 Reservoir Simulation
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This paper presents the results of two long-term horizontal well tests in the Troll field. Planning and evaluation of the tests and simulation of and consequences on field development are summarized. The two tests demonstrated significant production potential from 12- to 22-m oil columns and verified the pretest assumption that one horizontal well could replace four vertical wells.
The Troll field, in 300-m-deep-water offshore Norway, contains oil rims of varying thicknesses between the overlaying gas and the aquifer below. The field is divided into three main provinces: Troll West oil province, 22- to 26-m oil column; Troll West gas province, ˜12-m oil column; and Troll East, 0- to 4-m oil column. The oil in place in the two provinces with the thickest oil columns was estimated to be 155 and 440 × 106 std m3, respectively (Fig. 1).
The reservoir in the Troll field consists of dipping, highly permeable (3 to 10 µm2) clean sands interbedded with dipping micaceous and silty to fine-grained sands of significantly lower permeability (in the 1-µm2 range). The sands are highly unconsolidated. Calcite cementation occurs in all lithologies over the entire field. The calcites can be divided into two classes on the basis of their assumed lateral extensions. Regional calcites are thought to be sheets up to several kilometers wide occurring at boundaries between geologic zones, whereas local calcites are thought to be less extensive (1 to 100 m) and to occur within the geologic zones.
The highly permeable clean sands, C sands, are the target sands for the horizontal wells and contain 60% to 75% of the oil in place; the remaining oil is contained within the micaceous sands, M sands, which are hard to drain under the conditions in Troll. The dipping sands and the thin oil columns result in elongated target areas for the wells (Fig. 2).
The Troll reservoir, at the shallow depth of 1300 to 1570 m below mean sea level, is at hydrostatic pressure. The oil viscosity in Troll West varies between 1.3 and 1.8 mPa·s. Table 1 lists reservoir and fluid properties. In parts of the field, a residual oil zone exists below the oil/water contact (OWC). The relative permeability to water at 25% residual oil saturation (ROS) is low, from 0.05 to 0.40.
Oil production will be limited by gas coning, which results in a rapidly decreasing liquid rate, and the increasing water cut reduces the oil rate even further. In general, horizontal wells are known to improve productivity and to reduce coning problems compared with conventional vertical wells.1-4
For the Troll field, pretest simulation studies indicated that development of the Troll West oil province based on 500-m wells could be economically attractive. However, owing to such factors as 300-m-deep water, a highly unconsolidated reservoir rock, and a thin oil column, the development was considered to be a high-risk project. To confirm the horizontal-well potential and long-term behavior, and thereby reduce the risk involved, an 11-month long-term test with a 500-m horizontal well positioned 3 m above the OWC was carried out in the Troll West oil province during 1990.
The Troll West oil and gas provinces previously had been subject to studies aimed at commercial development of the large oil accumulation. However, no sound economy was found in these studies, which were based on the use of vertical wells. Encouraged by the good results obtained from the first horizontal test well, a new study was initiated that based a Troll West gas province development on the use of horizontal wells. The uncertainties related to the Troll West gas province development were assumed to be even larger than for the Troll West oil province owing to a need for increased well length and the thinner oil column. Therefore, a long-term test was carried out in the Troll West gas province immediately after the Troll West oil province test ended. The second horizontal test well was 800 m long and target depth for the horizontal section was 1 m above the OWC. This target depth required improved drilling accuracy.
The objectives of the first long-term production test with a horizontal well in the Troll West oil province were to gain experience and information on the most important uncertainties.5,6 The tests were intended to demonstrate the viability of drilling and completing a horizontal well in the highly unconsolidated Troll sands; to verify the theoretically expected productivity increase and reduced coning compared with those in vertical wells; and to observe time to gas breakthrough, water-cut development, critical oil rate trend, and possible hysteresis effects.
The experience and encouraging results obtained from the first test well were also the basis for planning and deciding to drill the second test well in the Troll West gas province and for performing a shorter, but similar, test program.
The objectives of the second test well were (I) to confirm the viability of extending the horizontal section from 500 to 800 m; (2) to improve drilling accuracy by use of pressure and resistivity measurements while drilling; (3) to verify pretest predictions of increased oil rate, delayed gas breakthrough, and reduced water-cut development; and (4) to evaluate oil production above critical rates (supercritical production).
The information from the two tests has been used (1) to verify the technique for improved drilling accuracy; (2) to calibrate the various reservoir simulation models, well and full field; (3) to adjust geologic and reservoir parameters through the history-matching process; (4) to verify relative permeabilities from the first history match by using them in the second test match; and (5) to establish more optimal development schemes, with reduced uncertainty, involving supercritical production in the Troll West gas province and longer wells in the Troll West oil province.
Figs. 3 and 4 show the ranges of oil rate and cumulative oil production from the pretest simulations for various geologic and reservoir sensitivities compared with actual performance of the two test wells.
Horizontal Test Well Locations
The following reasoning formed the basis for deciding on locations for the horizontal test wells (Fig. 1): (1) both wells are in areas that contain major amounts of assumed reserves of the region; (2) drilling a high number of development wells is planned in and near the two tested areas; (3) both wells are close to previously cored exploration wells - particularly important in the case of the uncored Troll West oil province well; and (4) comparison of vertical and horizontal well performance was facilitated because the horizontal test wells were close to existing tested vertical exploration wells.
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