QX® gives an unprecedented view of pore pressure variations in the subsurface for the explorationist. Its primary use is to show pressure contrasts and gradients, and to help in the risk assessment of prospects.
DrilSeis® utilizes two independent approaches to determine pore pressures and fracture gradients. Drilseis provides a comprehensive approach to project well planning and operations to efficiently identify lithology, porosity, and fluids to improve pre-drill estimates of pore pressure and fracture gradient volumes.
Some commonly asked questions:
- What is the difference between V-based and Q-based Pore Pressure; if they both measure pore pressure why don’t they give the same answer?
Neither technique measures pore pressure directly. They both derive estimates of pore pressure by measuring parameters derived from seismic data and relating them to pore pressure. The parameters for Q and V (frequency attenuation and moveout velocity, respectively) are independent of each other, may be affected by things other than pore pressure, and are subject to some error in their measurement.
- Frequency attenuation in seismic data is well known and I’ve seen absorption plots – what’s the difference between this and what you use for Q-based Pore Pressure?
It’s the same phenomenon. In our patented (pending) process the key relationships and the appropriate scaling and conversion operations are applied to the data in order to render the information about pressure, which is contained in the frequencies, with precision and accuracy
- What is the difference between Q-based Pore Pressure and QX?
They are both based on the same approach and, indeed, use the same algorithms. They are designed to serve different interests. QX does not come with a Frac Gradient Volume (it does come with a Seal Capacity and a Fluid Gradient Volume); its primary use is to show pressure contrasts and gradients and to help in the risk assessment of prospects (e.g. by comparing anticipated hydrocarbon column height with seal capacity). Pressure contrasts and gradients are of interest to the explorationist who wishes to know the geometry of pressure cells; whether pressures are influenced by or cut through structures (important for establishing the timing of pressuring versus structuring); leaking or sealing faults or shales; or the presence of pressure plumes or sinks. We do not recommend QX for well planning. Q-based Pore Pressure is part of the DrilSeis package. It is delivered with a V-based Pore Pressure Volume, as well as a Frac Gradient Volume. It is designed as part of a package to be used in the planning of wells to be drilled into high pressures, where an accurate assessment of potential pressures is important.
- Does DrilSeis require calibration wells?
No, but the quality of the result is improved if there are any available. For truly rank wildcat wells we have a number of default curves we can use, but even a well several miles away, which can be tied seismically to the prospect, will improve the quality of the prediction.
- How do you handle the centroid effect?
The SAIL Lithology and Porosity Volumes can be used to make an estimate of the extent of a porous reservoir. A reasonably conservative estimate of the centroid pressure can be had by taking the shale pore pressure (from one of the Pore Pressure Volumes – you’ll have to convert from mud weights) at a point about two-thirds of the way down from the top of the reservoir. The expected value of the pressure at any point in the reservoir can then be calculated by adding (or subtracting) the product of the pressure gradient and the elevation difference (between the elevation of the point and the centroid) to the centroid pressure. The Fluid Gradient Volume may also supplement this exercise: it is designed to distinguish between gradients associated with over-pressured shales and those associated with permeable gas- or fluid-filled reservoirs.
- How do you calculate velocities for V-based Pore Pressures?
We have a proprietary technique, which automatically picks the best moveout velocity to flatten the gathers at every sample, at every CDP. The technique is based on semblance technology and has the ability to distinguish between primary and multiple energy. We also have an error-rejection algorithm, which automatically promotes the most reliable velocities for inclusion in a final velocity volume.
- What are the error bars; what are the 90% confidence intervals?
Our approach is not a statistical one; it is deterministic. As such we cannot generate statistical measures for each calculation. In order to help determine the reliability of a given point (for example along a well path) we perform a search on the adjacent CDP’s (over a user-selected area) and retrieve the minimum and maximum pressures. By performing this search at regular intervals along the proposed well bore, bands showing the high, low and calculated pore pressure, along with the calculated fracture gradient are plotted for use in well planning.
- Can you prepare a drilling prognosis for me?
We do not prepare well prognoses or well plans. Our goal is to provide you with the critical information you need. By examining the Lithology and Porosity Volumes for the presence of reservoir quality rocks and especially those indicated to contain compressible fluids you may intelligently anticipate zones, which could strongly impact your drilling if you are significantly over- or under-balanced. By examining the Pore Pressure Volumes and the Frac Gradient Volume you may design optimal casing programs (we routinely provide pressure and frac gradient profiles along selected well paths). As explained above, accounting for a possible centroid effect is also straightforward with the information we provide.
- How does the presence of salt affect your calculations?
Salt has a fairly substantial impact on V-based pressure calculations. Near salt domes and walls velocities become erratic and it is difficult to get a consistent and reliable calibration. Beneath salt seals or pillows the data are often heavily influenced by multiple energy or simply too noisy to provide a reliable stack. We have techniques for rejecting error-prone or multiple-influenced velocities and we spend a lot of time and effort adjusting the parameters used in these techniques to get the most useable velocity data possible, but sometimes the amount of good information is not great. Frequency attenuation is not so greatly affected, especially around vertical salt features and, therefore, the Q-based pore pressures tend to be more reliable. There is oftentimes a fair amount of absorption around horizontal salt interfaces; we can account for this and regularly get very good results for Q-based Pore Pressure predictions from beneath salt.
- Should I use the maximum pore pressure values and the minimum frac gradient predictions when I am designing my mud and casing programs?
Because of the way we capture minima and maxima (see above) it is unlikely that both extremes would be present in the same well path. Our recommendation is that you should primarily plan for something less than the extremes, but that you should be aware of the possibilities and have contingency plans in place.
- Can you see gas chimneys?
The effect of gas is to reduce the velocity and to increase the absorption. Both V-based and Q-based Pore Pressure predictions will be affected in a positive direction (i.e. higher pore pressures). If we have well control for calibration or if the chimney is obvious we can correct for this effect.