Borehole-Stability Analysis for the Design of the First Horizontal Well Drilled in the U.K. Southern V Fields
- G.F. Fuh (Conoco Inc.) | E.G. Dew (Conoco (U.K.) Ltd.) | C.A. Ramsey (Conoco (U.K.) Ltd.) | Keith Collins (Conoco (U.K.) Ltd.)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling Engineering
- Publication Date
- September 1991
- Document Type
- Journal Paper
- 169 - 176
- 1991. Society of Petroleum Engineers
- 1.6.9 Coring, Fishing, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 4.2 Pipelines, Flowlines and Risers, 1.10 Drilling Equipment, 1.1 Well Planning, 4.1.2 Separation and Treating, 2 Well Completion, 1.6 Drilling Operations, 1.12.6 Drilling Data Management and Standards, 3 Production and Well Operations, 4.1.5 Processing Equipment, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 1.6.6 Directional Drilling, 3.2.5 Produced Sand / Solids Management and Control, 2.4.3 Sand/Solids Control, 1.8 Formation Damage, 2.2.2 Perforating
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This paper presents the method and results of a borehole-stability analysis conducted on the first horizontal well successfully drilled in the North Valiant field in the Southern North Sea. Previous drilling data obtained from the same area were analyzed by numerical Previous drilling data obtained from the same area were analyzed by numerical simulations of potential mechanical behavior of the wellbore. The magnitude of the in-situ horizontal stresses was determined by the analysis of fracture-stimulation data from wells in the fields. The directions of the stresses were derived from anelastic-strain-relaxation (ASR) measurements of fresh cores. Effects of hole azimuth on wellbore stability were evaluated. The analysis resulted in a design of optimum mud weights for maintaining borehole stability during drilling. This paper also discusses field results and well completions.
In the past few years, the oil and gas industry has experienced a remarkable surge of activity in horizontal drilling and completions worldwide. The demand for horizontal technology is high because it has several diverse applications; e.g., it can be used to increase well productivity, to produce low-permeability reservoirs, to avoid oil/gas contacts, to inject steam, to control sand production, and to produce thin hydrocarbon reservoirs. Well 49/16-PO5/03 was proposed as a horizontal well to improve the deliverability from the North Valiant 1 platform (Fig. 1). The plan was to drill a sufficient horizontal section that the probability of hitting a high-permeability sand dune in the Rotliegendes sandstone would be higher than the probability of hitting a sand dune with a vertical well. Well 49/16-8, an exploration well (Fig. 1), indicated that commercial gas production from these sand dunes could exceed the results expected in the North Valiant 1 platform area. However, subsequent production wells failed to find the same magnitude of production. Even Well 49/16-PO2/02, a twin of the exploratory well, failed to find the same highly productive intervals. It became apparent that a new method would be required to find the sand dunes in an economical and predictable manner. Hydraulic fracturing was tried on the best candidate but only marginal economics were achieved. In addition, the gas column above the gas/water contact is relatively thin (100 to 150 ft). A few weak barriers exist, and breaking into the water-bearing interval was a serious concern. Borehole stability in both the build and the horizontal sections of the well also was a concern because all the wells drilled in the North Valiant field had hole angles from 0 (vertical) to 50". Also, borehole stability would influence the mud weights used in the horizontal section and the weight and grade of the 5 1/2 -in. perforated liner to be run to complete the well. Questions on liner stability for overburden support during production were raised and evaluated before the horizontal well was drilled and completed.
The North Valiant field forms part of Conoco (U.K.) Ltd.'s V fields development in the U.K. southern gas basin. Gas is produced by means of two normally unmanned, 10-slot satellite platforms and sent by pipeline to the Lincolnshire Offshore Gas Gathering System terminal at Theddlethorpe. Drilling began in Sept. 1988, and the complex was brought on stream in Sept. 1989. Four wells were initially drilled through the northern platform, North Valiant 1. The production rates from these wells did not platform, North Valiant 1. The production rates from these wells did not meet the expectations from the exploration wells on the field. The reservoir rock of the southern North Sea is the Rotliegendes sandstone, which is composed of fine to medium sand grains. The dunal fades in the North Valiant area are very heterogeneous with an order-of-magnitude difference in permeability over short distances.
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