Monday, 8 May 2023

Session 1: Causes of Low Resistivity Pay (LRP)/Low Resistivity Contrast Pay  (LRCP) in Clastic Reservoirs

Low resistivity pay and low contrast pay have been long standing challenge in petrophysical evaluation for decades resulting in overlooking oil and gas opportunities due to low computed hydrocarbon saturation and consequently underestimating the reserves and/or bypassing pay zones.

The sources of low resistivity pay in clastics are classified mainly into three main reasons. First reason is related to microporosity in fine-grained (silty) sands, leading to actual high water saturation, but it is irreducible and hence water free hydrocarbon is produced. The second reason is underestimating of actual hydrocarbon saturation caused by presence of conductive minerals such as clay minerals and iron minerals particularly pyrite. The third reason is finely laminated sand/shale sequences, in which the thickness of the individual layers is much smaller than the vertical resolution of the resistivity tools.

Clay minerals are conductive due to their high cation exchange capacity (CEC) value. The clay with higher CEC has the more impact on lowering the resistivity reading. Clay can be distributed in the clastic rocks in four different ways: laminated, structural, dispersed and any combination of these style.

Low contrast pay occurs mainly when formation waters are fresh or low salinity, as a result, resistivity values are not necessarily low but there is little resistivity contrast between oil and water zones.

Session 2: Causes of Low Resistivity Pay (LRP)/Low Resistivity  Contrast Pay (LRCP) in Carbonates

In recent years, in most parts of the world, the conventional reservoirs are already “creamed”, where the best prospects have been already explored and involved into production. While the remaining potential of conventional plays is limited, the oil companies face the challenge of exploring more complex reservoirs that have been overlooked before or that were not considered economic to develop. These include the low resistivity and low resistivity contrast pays.

In most cases, Petrophysicists associate the concept of Low resistivity pay with laminated sand-shale sequences, but these challenges are not limited to these types of rocks.

The role of complex carbonates is becoming increasingly important. Economic rates from the intervals with hydrocarbon saturation less than 0.40 v/v are not uncommon.  In some cases, the low resistivity observed in hydrocarbon bearing intervals of such reservoirs is associated with a combination of microporosity/high residual water saturation and high salinity/low formation water resistivity.

Another challenge is represented by reservoirs where there is no clear resistivity contrast and saturation between water legs and hydrocarbon-bearing intervals despite saturation in both cases can be quite high. The reasons can vary from the trivial effect of deep mud filtrate invasion to the complex rock structure, for example unconnected mouldic or vuggy porosity. Complex mineralogy, the presence of conductive minerals, residual hydrocarbons and bitumen can also complicate the petrophysical interpretation.

The aim of sessions 1 and 2 is to deep-dive into the root causes of LRP/LRCP and their manifestation, their link to depositional environments and implications on Path to Explore, Discover and Develop of such reservoirs.

Day 2: Tuesday, 9 May 2023

Session 3: Evaluation Methods in Clastic Reservoirs

The word “complex” in the clastic world system fits in many dimensions. A whole different system can be examined, with wide range of scales, when looking under the microscope or to an outcrop. It might sound simple to some when dealing with three components, sand, shale and pore media However, nature has been continuously demonstrating challenging situations whenever the otherwise is presumed. This preludes into how can a complex system be evaluated and deciphered.

Advances in science is accelerating year by year for the last century and professionals are employing these advances with a cutting-edge technology to have a better understanding of the natural behaviors of rocks. There are almost unlimited combinations of mixtures of sand with shales, other minerals and/or porosity architecture. Then, it is our task to identify these combinations as it requires our skilled professionals to spot the challenges associated with it.

We mostly rely on our tool’s responses, experiments and understanding of the geological description to spot low resistivity or low contrast pay, which exhibits in many cases unique challenges. We always strive towards finding solutions to accurately characterize, evaluate and understand our matrix and porous systems.

Session 4: Evaluation Methods in Carbonate Reservoirs

Accurate quantification of fluids and their distribution in low resistivity and transition zone carbonates often poses a challenge to the interpreter. The heterogeneous pore structure systems of carbonate formations can be complex and challenging to evaluate. Dry oil production from low resistivity intervals, mixed flow production, or disparity between formation evaluation analyses and surface tests can lead to lower confidence in the downhole measurements or interpretation and increase the uncertainty of the calculated hydrocarbon volume in place.

If the oilfield is to be developed, it may take a long production time to validate the constituents, measurements, or assumptions of the Petrophysical model. Nevertheless, a variety of measurements and interpretation techniques may be exploited to mitigate these concerns.

Geoscientist working with such challenges, needs to have versatile skills and ability to efficiently integrate and interpret data from measurements acquired at different scales. Understanding the pros and cons of each methodology is a critical part of integrated data analysis. The pore-scale imaging and other DRP methods can assist in understanding the very localized pore systems. At the same time, may mislead the practitioner if samples are not covering all textures within the studied formation.

Fit for purpose SCAL analyses may reveal critical insights; however, the results need to be carefully evaluated and processed in order to be used in Petrophysical model. The Wireline and LWD survey are of lower resolution; however, are essential, and in addition to direct volumetric interpretation, can be used to establish a link (continuous logs) between the insights from point DRP/SCAL measurements and fluids distribution. The well logs analysis in LRP reservoirs may bring the misleading results when using conventional saturation equation. Considering the heterogeneity of carbonates textures, various tools and technology have been proposed to address the challenge of saturation evaluation in LRP/LRCP in carbonate reservoirs.

These range from better definition of the Archie paraments through the use of Resistivity Independent Saturation Evaluation has been proposed through the use of dielectric, NMR, and pulsed neutron logging.

All in all, the evaluation of LRP/LRCP carbonates requires good understanding of the reservoir and high-quality fit-for-purpose measurements.

Wednesday, 10 May 2023

Session 5: Core Analysis Role and Approaches in Identifying, Solving LRP/ LRCP

Low resistivity (LR) and low contrast (LC) resistivity pay reservoirs pose a major challenge to the industry when attempting to measure their conductivity versus different saturation units and capillarity. This can be rooted to the difficulty in recognizing their presence and quantifying their unlocked hydrocarbon potential. 

Laboratory core analysis (SCAL) techniques measures Electrical parameters (a, m and n) as a function of different fluid saturations. These experiments provide the necessary inputs to refine water saturation calculation from electric logs and the accuracy of reserves estimation are improved.
With excess conductivity experiments, effects of clay conductivity can be quantified. This allows us to correct the measured electrical parameters into m* and n*. These new parameters will improve reserves estimation.

Capillary pressure experiments, such as Mercury Injection Capillary pressure (MICP), provides additional understanding of porosity partitioning within the pore system. In addition, capillary pressure resistivity index (PcRI) experiments provides additional valuable information but is time extensive. Integrating the above information with petrography, XRD and SEM allows us to better understand porosity types; mineralogy, clay types, content and distribution and how they impact the acquired resistivity data. 

Though resistivity experiments can be time consuming and challenging in an environment where quick solution is required however if project is designed to objectively provide the desired results in an acceptable timeframe; the value of electrical measurements cannot be underestimated.

New techniques are on the increase and could provide new directions to identify, understand and resolve LRP/LRCP cases.
The challenge of describing and quantifying low resistivity and/or low contrast pay realistically depends on core availability, the quality of the core samples and sample representativeness. On top of that, sample size (i.e cuttings, plugs or while core) is another major factor to capture full heterogeneity since LRP/LRCP systems can depends heavily on them. 

This session will focus on building the understanding LRP/LRCP, current core analysis techniques and how we can leverage on new technology to provide new direction to identify new analytical techniques or modify current workflows.

Core Analysis measurements are the only “ground truth” that provide direct measurements on core samples hence it will continue to play a pivotal role in providing the necessary inputs to models for log interpretation.

Session 6: Digital Rocks: New Data and Methods

In the last years, the energy industry has been clearly taking advantage from novel, automatic approaches to data management, data analysis and data interpretation, in every aspect of its business: the “digital wave” is quickly changing the way we look at our assets.

Machine learning techniques provide the capability to process big amounts of heterogeneous data and to build data-driven models from examples; image processing techniques allow a quick and automatic analysis of many different geological images, including image logs, photographs of thin sections and core tomography images.

Together, they not only give robust and objective answers, but also contribute to maximize the value of company’s data and to cut down interpretation time.

Such novel approaches could benefit from the integration of conventional and high-profile measurements, in synergy with data-driven interpretation processes, to provide more realistic reservoir models, calibrated to the specific geologic framework under investigation and by exploiting valuable information derived from analogues and by honouring production history.

For instance, Bayesian Methods and Ensemble Modelling could provide intrinsic estimates of the uncertainty associated to the predicted properties.

In addition, fit-for-purpose image processing techniques could add significant insight to our knowledge of the reservoir fabric, thus allowing more robust estimates of the prospective productivity of each specific reservoir facies.

The aim of this session is to discuss the application of new digital technologies to the description and evaluation of low resistivity pays and its benefits, either in the assessment of the potentiality of new fields or in the revamping of mature assets and by-passed pays.