3D Geosteering for Accurate Placement of Horizontal Well in Malay Basin
Authors: Jamaluddin Syafiq , Hanalim Linda , Suwarlan Wahyudin , Santoso Gagok Imam , Permanasari Dian , Alang Khairul Anuar 
Published: May 2016

Abstract
This paper presents a case study of the first horizontal well in ADX field, Malay basin, which was drilled with an objective of maximising oil production from one of the minor reservoirs. In longer term, this well will be used as water injector once the reservoir pressure has been depleted, as part of the pressure management strategy in the field. To optimise the current production and the sweep efficiency at a later stage, a minimum of 500-m lateral length was planned for this well.
The target reservoir contains gas cap without any aquifer and is currently produced with natural depletion. The plan was to place the horizontal well as close as possible to the base of the sand and as far as possible from the known Gas-Oil-Contact, to delay gas breakthrough and to use it as water injector at a later stage. An upper TVD limit was determined in which the wellbore should avoid to keep certain distance from the gas cap. However, the formation in the target location was expected to have a dipping-up trend, which could significantly limit the vertical space to steer the lateral section and achieve the minimum target length.
A full suite logging-while-drilling measurement including deep directional resistivity which enabled 3D detection of approaching boundaries; combined with a proactive well placement method executed by collaborative experts from multidiscipline teams were used to address these challenges. As a result, the horizontal well was placed accurately within 0.5 m from the base of the reservoir along a 500-m lateral section; achieved with 3D geosteering decisions to avoid both the base and the upper TVD limit. Following the success of this first horizontal well in the field, another horizontal water injector well targeting a very thin reservoir was drilled successfully in the same field by applying similar methods and processes.

Introduction
ADX field is an elongated northwest/southeast-trending anticline that is fairly flat, with structural dip ranging from 1 to 3 degrees. The field is structurally well defined by high-quality reprocessed seismic in 1995 and 3D seismic in 2006. The structure comprises of an older east/west-trending fault system that compartmentalized the reservoirs at group K and below. The younger north/south normal faults exist at the flank of the field throughout all reservoirs.
The field was discovered in 1974 with the drilling of the first exploration well. To date, a total of eight exploration and appraisal wells have been drilled to define the structure and hydrocarbon accumulation. Currently, 110 development wells have been drilled and completed for production. Hydrocarbon accumulation was discovered throughout the H, I, J, K, L and M reservoirs. Major oil accumulation is mainly in the upper and middle group I sand; nonassociated gas (NAG) in the deeper I sand, as well as in the K, L and M reservoirs. Geologically, the reservoirs were deposited in a fluvio-deltaic depositional system—mainly channel and sand bars?with excellent reservoir qualities in the group I oil reservoirs. However, the reservoir's quality deteriorates with depth. The porosity ranges from 22 to 24% in group I, 12 to 14% in group K and less than 10% in group L, whilst permeability ranges from 100 to 1000 md in group I and 0.1 to 10 md in group K.
Stratigraphically, the group I reservoirs are composed of interbedded sequences of sandstones, siltstones and shales with widespread coal streaks that represent excellent correlation and seismic markers for subdivision of the sequence into genetic stratigraphic units. The section is dominated by a vertical stack of coarsening upward sequences, interrupted occasionally by blocky and fining upward channel sequences especially at I to F level. The group I sediments are dominated by fresh water palynomorphs; marine indicators such as foraminifera, shell hashes and dinocysts are very rare. Bioturbation generally is scarce, indicating that there were few marine incursions in the basin. Few paleosol intervals are recorded indicating intermittent exposure of the sediments. Overall, the group I sediments are interpreted to represent shallow water coastal fluvio-deltaic lobes along the southwestern margin of the Malay sedimentary basin during the Miocene period.