Third EAGE/AAPG Workshop on Tight Reservoirs in the Middle East

Many of the MWD and LWD systems that are utilized in the North America land market today were designed for drilling conventional wells and have limitations when applied to drilling and logging unconventional wells. An MWD/LWD system has been designed specifically for drilling and logging these unconventional wells. The system employs sensors and technology that are critical to efficient wellbore placement and address the needs of drilling and geology.

This paper provides insight into the importance of knowing the minerals present in the reservoir, particularly the total clay percentage. It is vital to know the types and percentages of clays present for effective reservoir treatment and analysis. During the evolution of a petroliferous sedimentary basin, the clay minerals contained in the rocks undergo a series of changes in composition and crystal structure in response to tectonics and sedimentation. The amount and type of clay minerals are a function of the provenance of clastic minerals and of diagenetic reactions at shallow and greater depth in different tectonic and sedimentary settings. Clay minerals can be used to infer tectonic/structural regime, basin evolution history, and the timing of various geologic events.

With an increasing demand of hydrocarbon resource from tight reservoirs, some of the exploration studies have been re-focused on the previously under-explored tight formations in Abu Dhabi.

The Middle Triassic to the Middle Jurassic formations, mainly Gulailah, Minjur, Marrat, Hamlah and Izhara from deeper to shallower, are the play fairways of tight hydrocarbon resource in Abu Dhabi. However, due to lack of systematic data integration, these tight formations were poorly mapped without a clear definition of their tight reservoirs. This paper is focused on re-mapping these tight formations on a regional scale and characterizing the tight reservoirs in the updated geological framework. The reservoirs were characterized in terms of geometry, quality and its controlling factors of deposition, diagenesis and hydrocarbon migration. This characterization was based on the unification of regional well correlation, reflection pattern interpretation of re-processed 3D PSTM seismic data and geochemical measurement results of cutting samples and fluid inclusions. As a result, a layer-cake to jigsaw-puzzle reservoir model is associated to the Middle Triassic to Middle Jurassic tight reservoirs.

Oil and gas exploration in carbonate reservoirs typically utilize conventional techniques in perforation, matrix acidizing stimulation and well testing. Currently, oil companies have moved to more marginal reservoir targets. Application of these techniques has often yielded disappointing results and tighter zones are often abandoned for more promising target intervals.

With increases in oil and gas production targets, operators are being forced to investigate more marginal reservoir layers for reserves and production potential. The real challenge facing oil industry today is that successful exploration requires a change in drilling, completion and stimulation techniques currently utilized by operators.

Marginal reservoirs can have lots of resource potential and reserves, but may require horizontal well drilling, and/or multiple stage hydraulic fracturing to achieve economic production targets. This type of strategy is more expensive than conventional method(s) and is proven in some circles as potentially risky and many of these risks must be addressed and mitigated. In an effort to reduce risks and costs associated with the exploration process, a new stimulation strategy has been adopted for tight intervals to explore and appraise these intervals using vertical wells prior to going to horizontal wells.

This paper presents and analyzes oil exploration well case study in which a new strategy has been applied and evaluated. The study also demonstrates that this approach can lead to new resource discoveries, better reservoir understanding and improved well and completion design for future appraisal wells. Moreover, the paper highlights some of the additional challenges that this strategy may evoke and shows how these challenges may be overcome in the future.

Unconventional resources have gained prominence after their potential to produce hydrocarbons has been proved. Production from these resources became economically feasible as a result of combining horizontal drilling and multi-stage hydraulic fracturing reservoir stimulation along laterals targeting tight rocks with favorable petrophysical qualities. The extraction of hydrocarbons from these source rocks requires a step by step process starting with basin mapping, drilling coupled with geosteering, adequate cementing of the lateral in the wellbore and hydraulic fracturing completion. Each of these steps has key ingredients that need to be understood for a successful tight reservoir well.

Managed Pressure Drilling techniques have been gaining acceptance in recent years overcoming the drilling challenges related to pressure while attempting to minimize formation damage. For wells with a narrow margin between the pore and fracture pressures, the challenge does not end when the drilling has concluded. After total depth has been reached, addressing the surge/swab effect while tripping and running casing without MPD can create the pressure scenarios that led to using MPD in the first place. The surge/ swab pressures for small hole sizes can be extreme and may result in influxes or hole instability on the trip out to pick up casing. Running casing in the hole, in many instances, can result in hydraulic fracturing of the formation as the casing nears or enters the open-hole section. The magnitude of the losses and additional stresses within the rock may prevent getting the casing to bottom. After the casing is in place, the ECDs from the cementing process may easily exceed the fracture pressures, which could result in losses, poor zonal isolation, and remedial cement repair. The overall well construction process has been optimized with the successful implementation of MPD and MPC.