Second EAGE Borehole Geology Workshop

Stress-induced features, such as borehole breakout and drilling-induced fractures, often observed on borehole image logs, reflect the orientation of the in situ stress in the formation. If a large database of stress data is available from a given region this may be used to create a “conceptual model”, forming a link between structural geology and geomechanics. This paper presents a conceptual model of the North African region based on data from 100 oil and gas wells across Algeria.

Stress orientations show an overall NNW-SSE trend, which may be linked to the relative motion of the African and Eurasian plates. Local fluctuations of the stress field occur on all scales, ranging from large regional structures over local faults to localized deflections near fractures. Relatively flat surfaces, such as formation boundaries, also deflect the stress field if associated with a significant mechanical contrast. Examples of all have been observed on image logs. Borehole breakout is systematically located in mechanically strong rocks, such as sandstone reservoirs, but absent from softer shales, indicating that stresses are partitioned with higher magnitudes and/or high deviatoric stress in reservoirs. This has implications for borehole stability as well as for completion applications, such as hydraulic fracture stimulation.

Borehole images from six Orlen operated wells in SE Poland were investigated for structural features. Wellbore instability features were used to calculate earth stress, borehole stability and fracture stress.

Silurian shales formed in a uniformly subsiding basin albeit with abundant earthquake activity, and later tilted en-bloc to W or S. WSW dipping faults were found in most wells. They can be assigned a Hercynian origin with general tectonic transport towards ENE, but recently reactivated, as we note stress-deflection in one of the wells, at Streczyn.

Densely spaced ENE and ESE steeply dipping fractures seem to constitute a set of strike-slip fractures in response to a general N-directed horizontal stress; alternatively, they formed by hydraulic tension and later slipped tectonically. The fractures remain sensitive to strike-slip in the current stress field.

Maximum horizontal stress is relatively high, placing the area in strike-slip regime. There is high horizontal stress anisotropy but due to rock strength and low fluid pressure, the vertical wells are quite stable to drill. The regime is favourable for drilling horizontal wells along the minimum stress axis.

Conventional approach is dealing with standard well logs and techniques and gives qualitative assessment of properties in complex formations like assuming fractures, secondary porosity, anisotropy of stresses and reservoir properties etc. Impact of special methods, such as wellbore imagers (WBI), can help to exceed those limitations and to significantly improve quality and reliability of petrophysical, geological and reservoir models. This gives us new approach to deal with complexity and every time to invert it into positive results. Several examples of this approach on different complicated cases related to Pannonian basin are presented in this abstract. They cover naturally fractured metamorphic schists, vertical anisotropy in clastic reservoirs, hostile borehole conditions and intervals with secondary porosity.

In carbonate reservoirs, production of hydrocarbons often depends primarily on the secondary porosity (fractures, faults and vugs). Detailed analysis of fractures network and texture might help determine the major types of porosity and their contribution in effective fluids flow. This in turn can be used to select the best productive zones and to predict the behavior of reservoir rocks in time of production. Qualitative and quantitative fracture and texture analysis can be performed using recently developed processing tools for Techlog. The previous interpretation platform allowed working only with wireline data. Nowadays, the fracture, texture and porosity processing workflow, can be also applied to LWD images. A pilot project was conducted using two types of borehole images acquired in a carbonate reservoir ( Borghi et al., 2013 ): Wireline FMI from a vertical well and LWD MicroScope HD from a nearby horizontal well. The study covered manual interpretation of both images and detailed analysis using new Techlog plug-ins: Fracture Analysis ( Maeso et al., 2014 ) and PoroTex ( Yamada et al., 2013 ). Results of the study are presented together with observations regarding strengths and limitations of the two images types and of the used processing tools.

Sediment particle size and its distribution are fundamental attributes of sedimentary deposits that provide key information for reservoir quality evaluation. Traditional grain-size analysis can only be obtained from laboratory testing on rock samples from cores. In the absence of core or suitable cutting information, borehole images and nuclear magnetic resonance (NMR) logs are basic input data to estimate grain size. Recently several approaches were proposed for grain-size analysis from NMR for depositional environment studies and expandable sand screen design. In this paper, we propose a new approach for the continuous microfacies analysis from pseudo grain-size distribution of borehole resistivity images.

The true size of particles is very difficult to be measured directly from borehole resistivity image, especially when the particle size is less than the tool button size; additionally, correction factors are different for conductive and resistive particles depending on the resistivity contrast. Based on the high resolution resistivity image, we assume that the resistivity of multiple button measurements represents the particle size relatively, similar to the principle of analyzing textural changes within clastic environments using electrical borehole images. The cumulative probability of each resistivity curve represents a similar statistic to that of grain size in laboratory experiments and indicates the different hydrodynamic conditions. In certain geological settings, the continuous microfacies analysis can be achieved from the cumulative frequency curve shape.

One case study were performed to verify this new approach in Fan delta depositional environments. The analysis results are consistent with drilling core data and provide the detailed microfacies information for sand unit correlation analysis in multiple wells. We find this method is robust in the fluvial system and conglomerate-related depositional environments, and but it is very challenging to distinguish the mouth bar and channel fill in a river-dominated delta ( Yang et al 2016 ).

The complexity of large-scale fault zones, such as might be observable on seismic, often makes it difficult to determine the orientation and sense of movement of the fault on an image log. This paper presents a case, in which an image log was acquired in a single well drilled through a seismic scale fault. Detailed interpretation of the image log helped resolve an ambiguity in the interpretation of the seismic data.

The image reveals that the fault zone is characterised by alternating intervals of featureless sandstone, breccia, steeply dipping bedding and fractures. The orientation of the main fault is ambiguous and both a “normal” and a “reverse” drag scenario can be envisaged. However, the deviation of the wellbore allows a 3-dimensional reconstruction of the fault zone geometry which, when compared to a classic fault core and damage zone model, renders the reverse fault scenario unlikely. The wellbore penetrates the fault zone at a low angle which results in an exaggerated length of the zone as seen on the image. This allows for an unusually detailed characterization of the seismic scale fault, which may increase our understanding of the role of faults as seals or conduits for flow in the reservoir.

The value of high resolution imaging data, i.e., core and borehole image data in energy industry projects has been the subject of several papers (e.g., Lagraba et al. 2010 ). Maximizing the imaging data value depends in the first instance on a clear definition of objectives to be achieved with this sometimes expensive form of data acquisition (e.g., CT imaging data). It is essential that the individual objectives of the project stake holders (geologists, reservoir engineers, etc.) are aligned with tool technology application specifications defined in company-wide standards. Focussed on project budget and in line with company data acquisition standards, an appropriate imaging service is subsequently designed considering data acquisition conditions and other supplementary logging services. After the successful data acquisition, auditable and most cost efficient cross-disciplinary workflows, which are outlined herein, track each of the imaging project steps from data processing to the close out with final integrated geological/petrophysical interpretation results. The seamless application of the interpretation results (e.g., absolute permeability from BHI) in reservoir modelling workflows ultimately demonstrate the added monetary value of high resolution imaging data for the energy industry.