The majority of polymer applications to date have targeted medium-to-high permeability sandstone reservoirs containing reservoir brine with moderate salinity and hardness. Polymer flooding for enhanced oil recovery has been field-tested in carbonates, but applications tailored towards oil-wet, low-permeability limestone rocks are uncommon. This paper contains results from laboratory core flood tests performed on four strongly oil-wet limestone rocks from the Al Shaheen field, off-shore Qatar. The rock samples investigated had 25–30% porosity and 0.3–28 mD permeability; the reservoir brine had a salinity of about 120,000 ppm of which 10,000 ppm were divalent cations. The bulk fluid testing involved fluid compatibility tests, long-term stability tests, as well as rheology measurements. Three commercial polymers were screened, one of which successfully passed all the screening criteria. Core flood experiments were subsequently performed to determine relative permeability, adsorption, inaccessible pore volume, as well as permeability and mobility reduction. The crude oil had a viscosity of 8 cP at reservoir conditions, yielding a slightly unfavourable mobility ratio for the baseline water flood. At concentrations of 500–1,000 ppm, the selected HPAM polymer gave rise to some mobility and permeability reduction, as expected, but it did not seriously plug any of the four low-permeability cores. The dynamic adsorption was very low, around 10–20 μg/g rock, the inaccessible pore volume was 15–20%, which is in line with industry experience, and the injectivity reduction was in agreement with ECLIPSE and UTCHEM simulations, which suggest that significant polymer degradation did not take place. The low adsorption is attributed to the strong oil-wetness of the rock. Since the polymer flows in the water phase, presence of a strong oil film likely prevents the polymer from adsorbing to the rock. The results from a subsequent water flood showed that inaccessible pore volume introduced by the polymer persists during the subsequent water flood; such a feature has yet to be incorporated in UTCHEM and ECLIPSE. In fact, the data revealed many features, which could not be adequately captured by numerical simulation tools. The main conclusion from the laboratory work is that polymer systems with good viscosifying power can be tailored to recover oil from low-permeability carbonate rocks, pushing the permeability limit down to around 1 mD.

3D pore-scale imaging and analysis provides an understanding of microscopic displacement processes and potentially a new set of predictive modeling tools for estimating multiphase flow properties of core material. Reconciliation and integration of the data derived from these models requires accurate characterization of the pore-scale distribution of fluids and a more detailed understanding of the role of wettability in oil recovery.The current study reports experimental imaging progress in these endeavors for a preserved-state carbonate core from a Middle Eastern waterflooded reservoir. Micro-CT methods were used in combination with novel fluid X-ray contrasting techniques and image registration to visualize the 3D pore-scale distribution of residual oil in mini-plugs. Segmentation of the registered tomograms and their differences facilitated estimation of the residual oil saturation. These predictions from digital analysis agreed reasonably well with laboratory measurements of oil saturation from extraction of sister mini-plugs and spectrophotometry. The tomogram segmentations provide additional information beyond this average value, such as the fractions of oil associated with macroporosity and microporosity.After the tomogram acquisitions, one of the dried mini-plugs was cut and SEM imaged at this exposed face to provide 2D images of fine features below the micro-CT resolution limit, such as the characteristic dimpled texture of asphaltene films on calcite surfaces due to their local wettability alteration in the reservoir. A new registration procedure was developed to embed the SEM images from the cut plug into the tomogram of the original uncut plug at their correct locations, so that this highresolution wettability information could be integrated into the 3D pore network description and correlated to the local distribution of residual oil.

The giant Al Shaheen Field in Block 5 and Block 5 Extension, offshore Qatar, contains a stacked sequence of thin Lower Cretaceous reservoirs associated with a complex array of subtle faults which influence dynamic reservoir behaviour in certain areas of the field. A multidisciplinary analysis that integrates 3D seismic, well data and a regionally developed structural model indicates that at reservoir levels, deformation occurred in an incipient low-displacement (lateral and vertical) strike-slip dominated regime, characterized by a complex pattern of predominately steep to sub-vertical branching fault zones. Seismic interpretation and well calibration indicates two major fault zones trending WNW-ESE and NNW-SSE, showing a dextral and sinistral sense of shear respectively. These fault zones are expressed as elongated depressions on structure maps and are formed by discontinuous arrays of en-echelon segments resulting in a spatial variability of fault distribution and behaviour. A secondary set of faults trending NW-SE and bounded between major WNW-ESE fault zones can be consistently mapped and is interpreted to represent either synthetic Riedel structures or normal faults. At deeper Permo-Triassic levels, continuous, well-developed fault lineaments are observed below the weaker Cretaceous lineaments. The interpreted deformation in the Al Shaheen Field can therefore be related to the development of shear zones associated with the reactivation of pre-existing deep-seated Infra-Cambrian basement anisotropies during the Cretaceous Oman Alpine 1 and Tertiary Zagros Alpine 2 tectonic events. The proposed structural model of the Al Shaheen Field provides a key input and a further calibration area to constrain the tectonic evolution of Qatar and of the entire Arabian plate with regards to basin evolution, timing of structuration, structural style and faults and fractures distribution on both regional and field scales. Introduction The Al Shaheen Field is structurally located within the Qatar Arch (Fig.1), a broad N to NNE trending Infra-Cambrian (610-520 Ma) structure belonging to the Central Arabian Plate tectonic mega province. The Qatar Arch developed as a "basement block" flanked by thick deposits of the "Hormuz series" and separating the Northern & Southern Gulf Hormuz Salt basins. The kinematics and the geometry of the early Qatar Arch are still speculative (Bell, 2004; Edgell, 1992; Harvey et al., 2012; Husseini, 1992; 2004; Johnson, 2008; Konert et al., 2001; Stampfli et al., 2001; 2002; Ziegler, 2001). In the region, basement-related anisotropies are interpreted to have formed as old as Proterozoic to Ediacarian time and to follow three main trends (Fig. 2): N-S (Nabitah),NW-SE to WNW-ESE (Najd) andNNW-SSE to NW-SE (Mesopotamian; Zampetti et al., 2009 and references therein). The Nabitah lineaments include the N-S trending Neo-Proterozoic deformation associated with the assembly of Gondwana. In the Arabian Plate, Pan African structures form a complex array of shear zones, which general young to the East. These are mainly reverse structures, which deformed during accretion of older micro-continents. The WNW-ESE to NW-SE faults are interpreted to represent a strike-slip system associated with the major Gondwana-scale continental shearing event know as the "Najd event". The Najd shear zones are vertically dipping and transect the entire crust. The prevailing steep dips of these structures are an important aspect in the subsequent reactivation during basin formation. Step-overs between Najd shear zones formed pull-aparts and pop-ups at many scales, mainly relaying over the NW-SE Mesopotamian trends.

Production logging in ultra-long horizontal wells has long been recognized to be extremely challenging, both in terms of data acquisition and data interpretation. This paper describes the planning, execution and lessons learnt of an incident free production/injection logging (PLT) campaign completed in twelve shallow horizontal wells – water injectors and oil producers – to support the long term reservoir management strategy of the Al Shaheen field, offshore Qatar. In this giant offshore oil field, the acquisition of even partial inflow production data is considered worthwhile. A production logging programme was therefore considered as essential. A logging campaign was undertaken in twelve wells using tractor technology as means of conveyance, in a cased hole environment. The key objectives of this campaign were to:Acquire dynamic flow data to improve the understanding of the reservoir dynamics.Identify where sub-optimum patterns can be improved to maximize ultimate recovery.Assess the tractor technology as a means of conveyance in long shallow cased horizontal wells. The campaign showed that although static data are essential in understanding the flow performance of a well, they cannot solely always explain the flow profile along the wellbore. The acquisition of dynamic data is essential to understand the well behavior. Results confirmed that in some long horizontal wells the flow profile takes place uniformly along the entire logged interval, whilst in others sub-optimum conditions such as cross flow and thief zones were identified. The paper describes how the data acquired helped to identify these latter conditions along with the repair opportunities to improve oil recovery. This campaign proved that tractor is a viable means of PLT conveyance in shallow cased horizontal wells by pushing the limits of the technology to greater depths than coiled tubing – up to 14,550 ft – with less disturbance to flow. To maximize success in future well re-entries, completion design has been reviewed in new wells, in order to make them "tractor friendly". Introduction The Al Shaheen oil field in Block 5 and Block 5 Extension, offshore Qatar, consists of a stacked sequence of low permeability carbonate and high permeability clastic reservoirs at relatively shallow depths around 3,000 ft TVD. Although the field was discovered in the mid 1970's its development commenced decades later, in 1992, when Qatar Petroleum entered into an Exploration and Production Sharing Agreement with Maersk Oil Qatar (1). Today the Al Shaheen field produces ca. 300,000 barrels of oil per day and cumulative oil production exceeds 1.4 billion barrels.

The Shuaiba reservoir of the Al Shaheen Field (Block 5 and Block 5 Extension, offshore Qatar) can be subdivided into a larger carbonate platform area located in the NE of Block 5 and an intra-platform basin (southern part of Block 5) that was filled in by a forced regressive wedge (FRW) system during the Upper Aptian regression. The FRW is characterized by clinoforms with a marly base that act as intraformational baffles or seals. Capturing these 3D geometries in detail, and specifically the exact location and nature of the platform-basin transition, is essential to developing the next generation geomodels for field development. Hence, an integrated study of core and log analysis, seismic forward modelling and seismic interpretation of a reprocessed data cube was conducted. As a start, a field-wide correlation grid of available core and log data allowed for an initial coarse understanding of the depositional geometries and defined an approximate location of the platform-basin transition. As a second step, the expected depositional geometries were forward-modelled in order to better understand possible seismic responses within the bandwidth of the available 3D seismic survey. A complication is the relatively small thickness of the examined reservoir elements that are close to the limit of vertical seismic resolution. Finally, seismic interpretation picked up on this groundwork and it was possible to clearly map the platform margin, as well as the onset of the FRW in 3D. The obtained result was a detailed 3D seismic interpretation of the Shuaiba reservoir geometries, where the interpreted elements are tied with a grid of core and log interpretations. The key elements in the successful Shuaiba interpretation was early adaption of fully integrated and iterative geological and geophysical workflows where forward modelling of seismic responses was important for hypothesis testing.