Reservoir Management Aspects of Early Waterflood Response after 25 Years of Depletion in the Valhall Field
Authors: Tjetland Gunnar , Kristiansen Tron Golder , Buer Kjell 
Published: February 2016

Abstract
Valhall is a fractured chalk reservoir in the Norwegian sector of the North Sea which has been producing under compaction drive since 1982. Continuous drilling has taken place the last 25 years to both develop the field and replace failing wells.
A water injection pilot led to sanctioning of a waterflood project which began injection in the crest of the field in early 2006. The predictions of Valhall waterflood performance need to cover a wide range of parameters including the fault and fracture pattern, the initial, present day and future chalk properties, the chemistry and temperature of the injected water and the influence of compaction, all likely to have an influence on injection sweep and recovery. The Valhall chalk is highly compressible and can be liquefied under certain conditions and waterflooding gives additional rock compaction. The fracture network has also been significantly impacted by stress changes, compaction and resulting incremental shear displacement. The ion content of the injected water may impact the microscopic sweep efficiency, water chemistry issues of scaling is also a concern.
The early data is revealing a potentially varying waterflood response in different parts of the crest, with a fractured region having an influence on sweep in the area of one injector and a more matrix dominated response in another area. Understanding these variations will influence the location and design of future injectors and the expansion of the water injection area.
The paper covers the depletion history to date including the problems and challenges we are facing and the way we overcome them. The early waterflood performance data gathered and initial reservoir response is presented with the initial interpretation as to likely waterflood performance in the field. The paper is split into three main parts. First we review the risk picture before waterflood. Then we present some of the early observations from the field. In the end we discuss our interpretation and draw conclusions based on the observations at this point in time.

Introduction & Field History
Valhall is an oil field located 290 km SW of Stavanger in the Norwegian corner between the Danish and British sectors. The field was put on production in 1982 and has produced around 80,000 BOPD the last 10 years. The STOOIP is estimated to 2.6 BStb (Barkved et. al,1 2003).
The development has been challenging primary due to the dynamic nature of the field. The field consists of two main reservoirs; the Campanian to Maastrchtian Tor formation and the Turonian to Coniacian Hod formation. ¼ of the in-place oil is located in the Hod formation. The remaining ¾ is located in the Tor formation. The two formations are separated by a low porosity (5%) hardground. The Tor formation porosity (30–50%) was in some places exceeding 50% before production start-up, i.e. the reservoir was consisting of more oil than rock. There are dense zones embedded in the high porosity Tor that can have lower porosity (15–25%). The weak compacting chalk matrix has caused chalk production (Kristiansen,2 1996), casing deformations and collapses (Kristiansen et. al,3 2000), drilling problems (Kristiansen,4 2004) and compaction induced changes in matrix and fracture permeability (Powley et. al,5 1992). Application of new technology has been fundamental in maintaining the current production performance.
The initial matrix permeability (typically a function of porosity) was in the 1–10 mD range. Fracture permeability was found in the crestal part of the field (Powley et. al,5 1992). Initial well test data in the crest indicated total permeabilities in the range from 3–5 times matrix permeability to some extreme cases showing 10–15 times the matrix permeability. Due to strong depletion most of this has now reduced towards matrix values. However, some areas have maintained permeability above matrix value for a longer time.
The weak reservoirs are resulting in significant compaction (exceeding 10 meters in places) and resulting in seafloor subsidence currently exceeding 5.7 meters at the central platform complex. Approximately 50% of the hydrocarbon recovery drive to date has been from the rock compaction.