Formation Damage Laboratory Testing – Cost Effective Risk Reduction to Maximise Recovery
Formation Damage Laboratory Testing – Cost Effective Risk Reduction to Maximise Recovery
By Ryan McLaughlin, Ian Patey and Justin Green, Corex.
In recent years the oil and gas industry has seen rising costs associated with various operational activities such as drilling, completing and treating wells. However, economic pressure is not the only challenge within the market, environmental responsibility and the trend towards deeper drilling has lead to many operators taking a total quality management approach to enable successful well operations. Any additional information that can be obtained to assist with operational decisions is much welcomed. Laboratory testing is viewed as a cost eff ective and low risk route to gather vital information in understanding the areas which may create risk during the life of a well. Appropriately structured testing programs including advanced interpretation techniques can have short, medium and long term benefi ts. Th is article will set out the main arguments to undertake laboratory assessments, and discuss some of the areas where the results can be particularly valuable.
Mechanisms which have an unfavourable economic impact can occur at any point during the life cycle of the fi led such as drilling, completion, production, injection, treatment, and stimulation. Th ese mechanisms which are termed as "Formation Damage" can manifest themselves in various ways, but fundamentally involve interactions between the reservoir (rock and fl uids) and the introduced operational fl uids and hardware. Drilling mud infi ltration, poor mud-cake clean-up, fl uid retention, fi nes mobilisation and pore blockage, fl uid incompatibility and precipitation, emulsions/sludges, removal of cement, clay swelling, and sanding are all examples of mechanisms which can have an impact on productivity or injectivity.
Laboratory testing can be performed to identify these damaging mechanisms, and with the correct interpretation useful recommendations can be made on ways of avoiding or removing them. Th e testing therefore becomes part of the quality management "Plan, Do, Study, Act" cycle: laboratory testing checks for problems or mechanisms, defi nes the options available for avoidance, tests the solutions for eff ectiveness, and provides feedback to aid in implementation. In terms of risk, the greater level of understanding can not only reduce risk, but add value to the planning process, as it is signifi cantly cheaper to experiment in the laboratory than the fi eld. A key aspect of laboratory testing is that it is direct measurement, whereas models are indirect or derived measurements; test data can therefore be used as inputs which consequently supplement or improve models. In addition, independent testing is key in the "calibration" of vendor recommendations on fl uids and hardware, allowing comparison across vendors, fi elds, and operators.
Testing to examine wellbore operations typically consists of preparing core samples to representative wellbore conditions, and simulating the operational sequences under consideration. Care must be taken throughout the process, to avoid any impact of the equipment or procedures on the outcome of testing. Equipment must not corrode, even when fl owing strong acid under HPHT conditions; the techniques used to prepare the samples (cutting, cleaning, drying, saturation, permeability measurement) must not create artefacts; and the conditions and sequence tested must be representative in terms of the fl uids and hardware being considered, exposure times, temperatures, pressures, overbalances and underbalances. Expert consultants assist with the test design (e.g. mud cake development, horizontal versus vertical core holder orientation, wellbore operational sequence to be evaluated) so that the required objectives are met and also to ensure that test results are not misleading. Th e output data from testing typically includes permeability measurements, fi ltrate loss volumes, production/injection plots, and sample photographs, which are all used in aiding conclusions.
After having performed a well-designed and executed test it is, however, just as (if not more) important to understand the results fully. Relying upon permeability alone creates a high risk, as in short core samples it is common for both pore restricting (e.g. drilling mud infi ltration, scale precipitation, fl uid retention) and pore-enlarging (e.g. clay fi nes removal, cement removal, saturation change) mechanisms to be seen. Th e combination of these can lead to increases, decreases, or no overall change in permeability, even though there are a number of mechanisms which could potentially cause problems in the fi eld. For example, in short core samples it is relatively easy to mobilise and remove high surface area clays, which will increase permeability, where in the fi eld increased transit distance and concentration as the particles move towards a smaller volume in the near-wellbore area can lead to signifi cant reduction inpore space. To reduce risk and increase understanding of results, interpretative geological analysis including scanning electron microscopy (SEM), x-ray diff raction (XRD), thin section, and innovative techniques such as cryogenic SEM are all used to examine samples before and after testing to understand the impact of the sequence tested. Th ese short core fl ood tests are informative and generate the inputs required to enable up-scaling for lateral simulation.
Heavy oil
With the current (and future) emphasis on non-conventional reserves, traditional testing techniques can struggle to adequately represent heavy oil reservoirs. Specialist sample preparation techniques have allowed the core samples to be prepared in a manner that does not impact on their integrity, for example avoiding removal of oil cement which can create unconsolidated and unrepresentative samples. Improvements and innovations in geological techniques have also allowed for visualisation of pore-lining and pore-fi lling fl uids without impacting on the integrity of the samples.
HPHT
High pressure, high temperature (HPHT) reservoirs, particularly tight gas, have also historically proved challenging to perform representative testing upon. Useful testing is especially vital in these fi elds, as any damaging mechanism can have a signifi cant impact on permeability, and therefore the economic viability of a fi eld. Identifying and avoiding damage before it occurs is essential in HPHT testing, and the testing needs to be performed at meaningful temperatures and pressures; the main innovation here is the design of equipment at Corex that allows wellbore operational testing to be carried out at temperatures of over 200ºC including (if required) humidifi cation of gas at reservoir temperature.
SRA of injection operations and production drawdown operations
Scale Risk Assessment (SRA) which can range from prediction to squeeze design, Corex independently evaluate scale formation and inhibitor selection. Utilising state of the art laboratory equipment and methodologies in combination with expert post test geological sample evaluation, scale inhibitor chemicals are evaluated for formation damage mechanisms. Inhibition life time is measured in the laboratory and optimised for fi eld squeeze application
High-rate gas
On the other end of the spectrum to tight gas is highrate gas, which has also posed problems in the past in terms of accurate control and measurement of rate over a large range of pressures. Corex have recently designed equipment that refi nes this to a level never before seen in reservoir conditions testing
Assessment for Halite rich reservoirs and well operations
Specialist Cryogenic SEM analysis techniques and preparation as well as integration with modelling criteria will assist in the assessment of well operations for Halite rich injection or production intervals. Full wellbore Abundance of pore filling and grain coating Kaolinite clay booklets. fl uid sequences are simulated under reservoir conditions (pressure and temperature) to closely mimic those of the reservoir in question, thus accurate representation. Damage mechanisms (such as precipitation or dissolution) can be identifi ed which will specifi cally address the changes in equilibrium experienced with Halite rich intervals.
Conclusion
To conclude, formation damage testing is highly sensitive to the techniques and equipment used. It is vital that each test meets its objectives, so having fl exibility in procedures could be viewed as more meaningful than having a "standard" procedure that provides comparable results that do not necessarily relate to fi eld conditions. It is relatively easy to perform low-specifi cation formation damage testing in an unrepresentative way, but more challenging to mimic wellbore conditions closely. Formation damage test results are known to vary from laboratory to laboratory based upon equipment, procedures, and parameters used, so it is important to consider the capability of the laboratory when interpreting results or putting them into context.
These examples of "challenging" scenarios help demonstrate that, if the tests and equipment are properly designed and implemented, and results are fully interpreted, independent laboratory testing can signifi cantly reduce risk in operational decisions.
