Successful geopressure modeling requires integration of data from diverse sources.

With shareholders expecting higher profitability, the conventional wisdom in the oil and gas industry has been to cut costs to the bone, primarily through reducing staff to the point that major staff reductions are almost an annual event in many companies.

There is a better way to make a substantial improvement in profitability that is getting surprisingly little attention in the industry today. That way is simply to mount a major attack on the cost of geopressure problems that affect drilling wells (the term geopressure is used to encompass the various stresses and pressures that affect drilling operations, including pore pressure, fracture pressure, effective stress, rock strength, etc.).

Foremost in this attack is the transformation of the sometimes-large amounts of diverse kinds of data available at a location into knowledge about geopressures as they affect drilling operations.

Cost


Fifteen years ago at pore pressure work shops and conferences, the cost of geopressure problems to the oil and gas industry was estimated to range from US $4 billion to $5 billion a year. Today, irrespective of multiple workshops, studies and conferences on the subject each subsequent year, estimates of the current geopressure problem costs to the industry are in the range of $7 billion to $8 billion! Why has our apparent progress been negative in dealing with this costly problem that plagues the industry?

The industry has made progress that has not yet been reflected in the annual cost figures, and this progress provides the direction of future trends that will have a positive impact on the problem.
A study of geopressure in more than 1,000 wells around the world has led to advanced software technology. That study, with a multi-year joint industry project organized through the Drilling Engineering Association to develop a best practice for predicting geopressures in deep water, has provided a great deal of insight in the direction we need to take to really make an impact on geopressure problems.

Technical issues

Knowledge of geopressures is important to safe and cost-effective drilling as well as to exploration success, where the evaluation of seal integrity is critical. The current approaches and methods used to predict geopressures prior to drilling a well in an area known to have abnormal pressures have many limitations.

For example, most methodologies center around the assumption that compaction disequilibrium or under-compaction is the primary source of abnormal pressures. From experience gained in working with hundreds of wells in all parts of the world, our consulting staff has documented many cases showing there are several other significant sources of abnormal pressures to include hydrocarbon generation, thermal expansion, tectonic forces and hydrodynamic effects. Failure to take these effects into account has resulted in many occurrences of drilling problems that have been serious enough to require abandonment of the well above the target and/or a costly sidetrack.

Another common assumption is that pore pressures are the same in adjacent sand and shale formations. While this is often the case, this can be a disastrous assumption. There are many instances of drilling into a sand formation that has significant structural relief where the effect of hydrodynamics cause high abnormal pressures resulting in well fluid influx and/or blowout.

Seismic velocity data are frequently used for estimation of pore pressures and can give a false sense of accuracy when we forget that resolution of seismic amplitudes is typically coarse, with 200 ft (61 m) considered exceptional and 500 ft (152 m) considered good. As many drilling engineers can testify, a lot can happen in a 200-ft (61-m) interval.

More significantly, all seismic velocities are not created equal when it comes to their use in geopressure prediction. For example, stacking velocities are often greatly inferior to velocities determined from tomographic inversion in terms of depth resolution and accuracy. While the processing costs for higher-quality velocity data can be significant, the savings of 1 or more days required to circulate out a kick using a rig with a day rate of $250,000-plus can more than justify that cost.

Fluid movement over geologic time through a reservoir with complex faulting is important to geopressure prediction and difficult to determine. The effect of pressure compartments is difficult to evaluate when looking only at a single 1-D model. The difficulties that arise from inadequate knowledge of geopressures have become more prominent with drilling in the deep water, where wells are very costly and the margin between pore pressure and fracture gradients can be small.

Seismic data, petrophysical data from logging while drilling and wireline, geologic data, and drilling data from adjacent wells all are necessary inputs to this work.

Future trends

Future approaches are going to require an understanding and evaluation of the total geologic environment in three dimensions to be successful. Geopressure modeling at the basin scale will be increasingly important, and in general more time, cost and effort must be applied to the process of geopressure prediction.

With wells in deep water costing tens of millions of dollars, it is foolhardy to use simple equations or overlay curves as the sole planning tool for predictions of such importance. The complexity of the issue requires a team effort that includes both the geoscientists and the engineers. The need to improve drilling safety and reduce costs can provide the demand that can result in the various interested parties, data holders and analysts being drawn together to provide the necessary information to build a 3-D geopressure model. Only a compelling demand by management and drilling teams for data integration and cooperation in the interest of saving money and increasing safety can bring these diverse disciplines together for successful geopressure modeling projects.

New technology in the form of geopressure-related computer software has to be applied in order to make an impact on reducing costs and increasing safety.

Such a software system needs to:

• Provide a facility for a multi-disciplinary team of engineers and geoscientists to contribute to a project where all data available is integrated and used.
• Provide a facility to evaluate prospect seal integrity to analyze target locations early in the prospect evaluation stage.
• Predict geopressures at the basin scale.
• Combine 1-D well models and 3-D basin models with 2-D and 3-D seismic data into a single well project.
• Be used for pre-drill geopressure prediction and wellsite monitoring so that the prediction can be updated while drilling. This is essential given that the best possible predictions are still subject to numerous unknowns and assumptions.
• Fully integrate the functions of pore pressure analysis and wellbore stability analysis into a single system in both pre-drill planning and wellsite monitoring modes of operation.
• Provide a seamless interface to data from mixed systems and a variety of datastores (eg. Landmark and Geoquest).
• Build a fully relational geopressure database that can be used for future analysis. This will constitute a significant corporate asset.
• Include a programming language so that users can build in new and different models that can work on any data used by the system.

• Have a facility to provide quality control and correction/normalization for all data used to meet the unique requirements of geopressure analysis, to include:

• Correct seismic velocity data for all bias - anisotropy, etc.
• Correct and normalized petrophysical data for effects of temperature, salinity, anisotropy, etc.
• Have general inversion and calibration facilities so that all known data can be used to calibrate the various models.
• Be able to consider hydrodynamic (e.g. centroid and buoyancy) effects for those cases where geologic structure results in geopressure differences between dipping sands and shales.
• Have the ability to evaluate and quantify uncertainty in predictions.
• Have the ability to display all data and models in a manageable 3-D view that can easily be exported to the modern visualization centers being used today.

Many of these capabilities are available today in the form of software. Because of inertia, lack of management awareness, penny-wise/pound-foolish budgets and loss of experienced industry personnel, the actual use of these tools can is limited.

These software tools require experienced users; use by inexperienced users that are unfamiliar with the theory and assumptions built into the tools can be dangerous. We need a crash effort to educate and train the current generation of engineers and geoscientists in this important technical area so that these costs can be reduced industry wide. This will require an investment of time and money by the industry, but I am confident it will be but a small fraction of the amount lost by the industry each year to geopressure-related problems.

The next generation of technology will allow operators to leverage more of their data, get geoscience and drilling teams working together, and move beyond a single location. It will allow operators to focus both on the individual well and field scale, growing with operators as they drill more wells.