Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Earth science
Reexamination Certificate
2002-07-26
2004-01-20
Barlow, John (Department: 2863)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Earth science
C367S038000
Reexamination Certificate
active
06681185
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to seismic signal processing and, more particularly, to apparatus and methods for improved interpretation of seismic data to more accurately identify geopressures, seismic velocities, and/or other formation attributes.
2. Description of the Background
When planning the drilling of a well, the drilling engineer must make decisions about the mud program, the casing program, and the like. The consequences of such decisions are significant and impact the likelihood of success of the well, the cost of the well, the environment, and even the safety of the well personnel. For instance, if the mud selected or used at any particular depth is too light to hold the actual well pressure, or if other pressure control equipment utilized is inadequate, then blowouts, loss of life and property, environmental damage, and the like may occur. Costs of blowouts may also include environmental remedial costs, loss of production, and relief well costs. If a blowout occurs, well control specialists can be quite expensive, costing millions of dollars per well. On the other hand, if the mud selected is too heavy, then loss of circulation may occur with costly consequences such as kicks, underground blowouts, stuck pipe, and the need for side tracks. As well, damage to the formation may occur that may reduce or limit the well's productivity. Additional control equipment for handling pressure is costly and the over-engineering of a well can add significantly to the well costs. If the anticipated pressure does not materialize, then this money is wasted.
It is therefore desirable to keep drilling mud weights as light as possible to most economically penetrate the earth. It is also desirable to know what pressures are expected so that suitable but economically reasonable pressure control equipment is available on site. The required mud weight will typically vary with depth during the drilling of the well. For the reasons discussed above, mud weight is carefully monitored and may be increased during drilling operations to compensate for geopressure.
It is often desirable to set casing in a borehole immediately prior to encountering a pressured formation and then to increase mud weight for pressure control during further drilling. Setting a casing string which spans normal or low pressure formations permits the use of very heavy drilling muds without risking breaking down of borehole walls and subsequent lost mud. On the other hand, should substitution of heavy drilling mud be delayed until the drill bit has penetrated a permeable overpressurized formation (e.g., sandstone), it may be impossible to remove the drill string without producing a blowout or otherwise losing the well.
Geopressure conditions conducive to blowouts occur, in general, if fluids become trapped in rock and must support some of the overburden weight. As an example, there may be an earth formation of high porosity and high permeability, or a series of such formations, within a massive shale formation that is relatively impermeable. Fluid pressure of fluids trapped within such highly permeable formations (which usually are sands) may increase as the weight of overburden increases during sedimentation above the shale formation. When such formations are penetrated, the large pressure gradient into the borehole can easily result in a blowout.
To make the well program decisions, the drilling engineer typically relies on data from offset wells, assuming offset wells have been drilled. If there are no offset wells, then the decisions may be made in the light of significant geologic uncertainties. Even if there are offset wells, assumptions are made to the effect that the future well will be like the offset well. However, drilling rarely goes just like the offset in spite of the best correlation efforts. For instance, pore pressures, and the attendant possibility of blowouts and/or extra gas production, can vary significantly within even the same field and within close distances of offset wells. Gas pockets with high pressures may be encountered in one well that are not present in a closely adjacent well. For instance, in some fields that normally have low geopressures, gas pockets may exist but may be rare so that when encountered the rig crew may be unprepared.
Well logs are sometimes used to analyze offset wells to determine, for instance, pore pressures at the offset well in the hope that the well to be drilled in the future will have the same properties. As discussed above, due to differences from well to well, the results may not be sufficiently accurate or similar enough to avoid significant problems, set casing at the appropriate positions, and/or to efficiently produce the hydrocarbons. The analysis for pore pressures involves use of well logs such as gamma ray, velocity, and spontaneous potential well logs to select corresponding shale points in a sonic or resistivity well log. A normal compaction trend line is then determined and the departure of the sonic or resistivity data from the trend line is used to compute pore pressures.
From the drilling engineer's point of view, it would be very highly desirable to accurately predict pore pressures from seismic data. In this way, the data could be specific to each well rather than after the fact and typically a variation from the values predicted by offset wells. Therefore significant efforts and attempts have been made in the past by those skilled in the art to utilize seismic data for such purposes. Unfortunately, attempts to predict pore pressure from seismic data have been relatively poor. In the prior art, seismic velocities are determined from seismic data utilizing normal move out techniques. From the seismic velocities so determined, as discussed in the subsequently listed patents, the normal compaction trend lines have been determined and pore pressures computed. However, while general trends may be seen from such analysis, the seismic velocity determined by prior art seismic data analysis techniques does not have sufficient resolution accuracy to permit actions to be taken at optimal well depths to most efficiently drill and produce the well.
The following representative patents show attempts to utilize or improve upon the move out velocities derived from seismic data for determining pore pressure as per the prior art:
U.S. Pat. No. 3,898,610, issued Aug. 5, 1975, to E. S. Pennebaker, Jr. discloses methods of geopressure assessment in an area proposed for drilling: first, perform a seismic observation (using a common midpoint (CMP) method as illustrated in Pennebaker
FIG. 1
) to determine average seismic velocity as a function of depth by move out techniques. Next, compute interval transit time as a function of depth, and then compare these observed interval transit times to putatively normal interval transit times as illustrated in Pennebaker. Depths where observed interval transit times are greater than normal indicate lower-than-normal velocity and inferentially greater-than-normal porosity and thus geopressured fluids. Putatively normal interval transit times are either (I) directly measured in a borehole in the general area which encountered only normal pressures during drilling or (ii) computed by following an expression for seismic velocity V (feet/second) as a function of depth D, with D measured in feet from a location of known seismic velocity.
U.S. Pat. No. 6,374,186B1, issued Apr. 16, 2002, to Dvorkin et al., discloses a method for overpressure detection and pore pressure change monitoring in subsurface gas, liquid hydrocarbon, or water reservoirs from compressional- and shear-wave measurement data. As part of this method, one or more Poisson's ratios are determined from field-based measurement data and are then compared against known Poisson's ratio values representative of the particular subsurface formation type. By applying a Poisson's ratio—pore pressure criterion that is appropriate for that type of formation, an overpressure in the formation is
Greve John F.
Pankratov Aleksey B
Young Roger A.
Barlow John
eSeis
Le Toan M
Nash Kenneth L.
LandOfFree
Method of seismic signal processing does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method of seismic signal processing, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method of seismic signal processing will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3247490