The Haven Oil Field: Development of a Tiny Marginal Field With Horizontal Wells
Authors: Target P.L. 
Published: December 2007

Summary
The decrease in large discoveries highlights the need for the industry tofocus on safe, low-cost, high-efficiency development of smaller fields. Thispaper describes the development and economic success of one tiny oil field inthe North Sea with horizontal well technology and reuse of existing minimalfacilities. Emphasis is placed on how predevelopment reservoir simulationpredevelopment reservoir simulation studies influenced the design and drillingof wells. The reservoir model results were the basis for economic justificationof the project. The paper also compares actual reservoir performance withpredictions. performance with predictions.

Introduction
The Haven oil field awaited development for 8 years after its discoverybecause recovery of its reserves of 0.62 × 10-6 m3[3.9 million bbl] was risky andthe economics were marginal. Water coning, giving rise to early waterproduction, was expected from vertical wells. Also, initial volumetricestimates had to be reduced as a result of delineation drilling. Unocal Netherlands' successful redevelopment of the Helder field with horizontal wellsspurred the company to apply the technology to fields with similar productionproblems. This led to a reappraisal of problems. This led to a reappraisal ofprevious development plans for Haven. previous development plans for Haven. Themain advantages of horizontal wells in developing fields with thin oil columnsare the increased oil recovery resulting from an improved sweep efficiency, higher PI resulting in lower drawdowns, and lower water cuts because coning isreduced. The reservoir shape, high permeability, and thin oil column made the Haven oil field a prime candidate for horizontal well development. Thereservoir was modeled in 3D with implicit blackoil reservoir simulationsoftware to compare the expected performance of horizontal and vertical wells. The performance of horizontal and vertical wells. The results showed that morethan two conventional wells would be required to recover the same reserves astwo 450-m [1,476-ft] horizontal wells drilled along the crest of the structure. This meant a small two-slot wellhead facility could be installed instead of alarger six-slot platform, thereby reducing the investment required andincreasing the economic margin. Such a facility was available in the form ofthe Helder B satellite platform. which was about to be abandoned and platform.which was about to be abandoned and had been originally designed forrelocation. Justification and approval of this project were based on thereservoir model results.

Field History and Description
The Haven oil field is a very small accumulation located in Block Q/l of the Dutch North Sea, about 100 km [62 miles] north-west of Amsterdam (Fig. 1). Itlies 5.8 km [3.6 miles] north of the Helder field in 27 m [89 ft] of water.
The field was discovered in 1980 by Well Q/1-8, which penetrated 16 m [52.5ft] of pay and tested at a rate of 525 m3/d [3,300 pay and tested at a rate of525 m3/d [3,300 BID] fluid. Water was produced almost immediately, and the cutrose to a maximum of 35% within 22 hours. Therefore, from the beginning it wasapparent that water coning would be a problem in any conventional developmentof such a thin oil column. Structural mapping problems were encountered in 1981when Well Q/1-15 was drilled to prove reserves 1.6 km [l mile] to thesoutheast, but penetrated the structure deeper than expected, 14 m [46 ft]below the oil/water contact (OWC). Two further delineation wells, Wells Q/1 -20and 21, were drilled from a central template in 1985 and 1986, with theintention of keeping them as future producers. Based on the results of thesewells, the original estimate of stock-tank oil initially in place was reducedby 50%, from about 3.8 to 1.9 × 106 m3 [24 to 12 MMSTB]. With the consequent dropin estimated reserves to only 0.62 × 106 m3 [3.9 MMSTB], the project becamemarginal. The structure is a long, narrow, low-relief anticline of friable, Lower Cretaceous Vlieland sand at a depth of 1600-m [5,250-ft] subsea (Fig. 2). Table 1 summarizes the reservoir and fluid properties. The oil column has amaximum thickness of 21 m [69 It] and is underlain by water over the entirearea of closure. This aquifer is common to all the Vlieland fields in Block Q/1. Vertical and horizontal permeabilities in the reservoir vary from 1.5 to 10darcies [1.51 to 9.9 m2], and there is little anisotropy. The sand is generallyhomogeneous and clean, but in the top 7 m [23 ft] of the northern part of thefield up to three shaly/silly sections of 1 to 2 m [3 to 6 ft] in thicknessexist. Development of the field was proposed in early 1986, using a four-pileunmanned platform and two vertical wells; however, the platform and twovertical wells; however, the subsequent fall in oil price caused the project tobe shelved. The main factors that led to the economic development of the Havenfield were the availability of two new technologies: the small tripod towerplatform (TTP) and horizontal drilling.
JPT
P. 496