Patrick J. Hooyman

Pore pressure prediction using well-conditioned seismic velocities

Abnormal pore pressures are encountered worldwide, often resulting in drilling problems such as borehole instability, stuck pipe, lost circulation, kicks, and blow-outs (Dutta, 1997). To optimize the choice of casing and mud weight while drilling abnormally pressured formations, a pre-drill prediction of pore pressure is required. A pre-drill estimate of pore pressure can be obtained from seismic velocities using a velocity to pore pressure transform calibrated from offset well data. However, velocities obtained from processing seismic reflection data often lack the spatial resolution needed for accurate pore pressure prediction, due to assumptions such as layered media and hyperbolic moveout. In addition, the uncertainty in velocity is often not quantified. In this example from the Gulf of Mexico, seismic velocities obtained using reflection tomography are combined with well data to produce a refined velocity field that honours the available well information. The refined velocity field is then used to predict pore pressure.

From pore-pressure prediction to reservoir characterization: A combined geomechanics-seismic inversion workflow using trend-kriging techniques in a deepwater basin

To optimize drilling decisions and well planning in overpressured areas, it is essential to carry out pore-pressure predictions before drilling. Knowledge of pore pressure implies knowledge of the effective stress, which is a key input for several geomechanics applications, such as fault slip and fault seal analysis and reservoir compaction studies. It is also a required input for 3D and 4D seismic reservoir characterization. Because the seismic response of shales and sand depends on their compaction history, the effective stress will govern the sedimentary seismic response. This is in contrast to normally pressured regimes, where the depth below mudline (or overburden stress) is typically used to characterize the compaction effect.

Use of reflection tomography to predict pore pressure in overpressured reservoir sands

Summary A pre-drill estimate of pore pressure is important for drilling in overpressured areas. In this presentation, seismic velocities obtained using reflection tomography are used to predict pore pressure over an area which contains several distinct pressure cells with pressure differences as large as 3000 psi. The velocity to pore pressure transform used is calibrated using an extensive data set from wells in the area.

Predicting Reservoir Compaction and Casing Deformation in Deepwater Turbidites using a 3D Mechanical Earth Model

Production from deepwater turbidites is made more challenging by geomechanical problems arising from overpressure, reservoir compaction, casing failure, etc. The impact of reservoir compaction on field development is best examined early in the life of the field, when there is an opportunity for proactive planning. The interaction between heterogeneous reservoir layers, well trajectories and stress changes due to depletion is a complex 3D problem; it is best solved using a 3D approach. This paper illustrates the use of a 3D MEM (Mechanical Earth Model) to quantify reservoir compaction, casing failure and surface subsidence for a deepwater Gulf of Mexico turbidite. Reservoir flow simulation data are used to evaluate stress changes, reservoir compaction, casing deformation and surface subsidence resulting from production. Geomechanical hot spots, where reservoir compaction is predicted to be greatest, are identified to allow production strategy to be optimized. Several possible modes of production-induced casing failure are examined. The likelihood of casing failure is then assessed to facilitate development of strategies for either avoiding such risks, or reducing their impact. This work shows the importance of developing a 3D MEM early in field development. In high-porosity, pressurecompartmentalized sandstones, quantification of reservoir compaction is essential for planning the casing and completion integrity through the life of the reservoir. Introduction Many oilfield projects today are challenging because of geomechanical problems arising from overpressure, wellbore instability, reservoir compaction, casing failure, sanding, surface subsidence, fault reactivation, etc. The use of a 3D MEM allows all information related to the geomechanics of drilling and production to be captured, including in-situ stresses, rock failure mechanisms, rock mechanical parameters, geologic structure, stratigraphy and well geometry. Once constructed, this model can be used to identify geomechanical problems and to devise contingency plans for handling them before the well is drilled. The use of a 3D MEM for predicting reservoir compaction from pressure depletion, plus its associated impact on casing deformation and surface subsidence, is demonstrated for a complex turbidite reservoir. Fig. 1 shows the steps involved in building a 3D MEM, while Fig. 2 illustrates the construction of a 3D MEM using well logs and seismic horizons. Fig. 3 shows how the MEM may be updated using data acquired while drilling a well. Fig. 1 Steps involved in constructing a 3D MEM. Fig. 2 Construction of a 3D MEM. Well logs are combined with seismic horizons to build the model.

Determination of in-situ stress and rock strength using borehole acoustic data

In-situ stress, pore pressure, and rock strength are required for the analysis and prediction of geomechanical problems encountered in the petroleum industry. While vertical stress, pore pressure, and minimum horizontal stress can be estimated using standard geomechanical techniques, the determination of maximum horizontal stress and rock strength are more difficult. This paper describes the use of sonic data to determine maximum horizontal stress and rock strength using data from a well drilled through poorly consolidated sandstone. The maximum horizontal stress is determined from estimates of the three far-field shear moduli, corresponding to the principal stress planes. These are estimated from the dipole flexural waves and Stoneley wave. Using sonic data, the rock strength is determined from the variation in the velocity of polarized shear waves as functions of azimuth and distance from the borehole.

Well‐constrained seismic estimation of pore pressure with uncertainty

Summary A quantitative predrill prediction of formation pore pressure with uncertainty is needed for safe, cost effective drilling in overpressured areas. This paper describes the use of a 3D probabilistic Mechanical Earth Model (MEM) that combines well data with seismic velocities to predict pore pressure and uncertainty. Application is made to an overpressured area in the Gulf of Mexico. Parameters in the velocity-to-pore-pressure transform are estimated using seismic velocities plus density logs, pressure data, and well velocities obtained by inverting time-depth pairs from checkshots in the area. A prediction of pore pressure and uncertainty is made by sampling the region of parameter space consistent with available well data.

Regional trends in undercompaction and overpressure in the Gulf of Mexico

Summary Shallow water flow and overpressure, arising primarily from rapid sedimentation rates generated by the Mississippi River depocenter, represent major drilling hazards in the deepwater Gulf of Mexico. In this paper, checkshots released by the Minerals Management Service (MMS) for the Gulf of Mexico are inverted for velocity versus depth below mudline, then kriged to populate a 3D Mechanical Earth Model (MEM) with both velocity and expected uncertainty. The 3D velocity cube thus obtained is used to infer regional variations in overpressure and undercompaction of shallow sediments.

An introduction to this special section: 3-D seismic interpretation

The importance of 3-D seismic to applied geophysics in the 1990s is dramatically illustrated elsewhere in this issue, in the pages dedicated to the awards to be presented at SEG’s imminent Annual Meeting in New Orleans. The Honors and Awards Committee, for the second straight year, has felt it necessary to award multiple Special Commendations for contributions to this technology and to publish its own reasons for the awards (in addition to the individual citations) which is strongly implies, again for the second straight year, that many other individuals and/or groups are also likely to be honored in the near future for still other work in this vital area. Such recognition is extraordinary, and probably unprecedented, for a technique that was strictly in the research domain 20 years ago and has been routine for only a decade or so.

A Three-Dimensional Seismic Survey Applied to Field Development in Williston Basin: ABSTRACT

The Medicine Lake field of Sheridan County, Montana, was discovered in March 1979. In October 1981, a mini-3-D seismic survey covering 2.5 mi2 (6.2 km2) was acquired over this field in order to facilitate development drilling by delineating the fields reservoirs and obtaining a more accurate image of the subsurface structure. A multiline system, consisting of 240 geophone groups distributed on 8 lines, was used. The energy source was shothole dynamite using 5 lbs (2.3 kg) charges at 250 ft (46 m). The shotpoints were arranged in a cross pattern with extra shotpoints included to provide necessary control on the weathered zone. The average subsurface coverage was 600%, with CDP bins 165 ft (50 m) square. Prior to the actual shooting, a computer simulation of the resulting fold was performed to verify the field geometry. The entire survey was recorded in one day with no movement of the geophones, thus minimizing costs. The data volume was processed in preserved amplitude through 3-D migration and 1-D inversion. The subsurface image was substantially improved by the 3-D migration process. The advantages of this enhanced focusing ability are particularly important when attempting to delineate the lateral extent of reservoirs and detect lithologic variations. The Medicine Lake field is located on a structural high, although there are stratigraphic implications for several of the producing zones. The interpretation of the data therefore focused on both structural and stratigraphic features. The Medicine Lake structure is prominently displayed on the Winnipeg event, showing a closure in excess of 180 ft (55 m). Several reflectors near the base of the Red River interval terminated against the Winnipeg event, indicating that this structure was a high in Red River time. Discontinuities in the Cambrian and Precambrian reflectors suggest that the Medicine Lake structure is a result of basement faulting. The objective of the stratigraphic interpretation was to outline zones of possible porosity, particularly in the Madison and Red River intervals. The horizontal and vertical inverted sections were particularly useful for ascertaining the location and lateral extent of those anomalous zones. The results correlate well with known production, and should aid in the location of future development wells. End_of_Article - Last_Page 541------------

Three-Dimensional Seismic Survey Applied to Field Development in Williston Basin: ABSTRACT

The Medicine Lake field of Sheridan County, Montana, was discovered in March 1979 by the drilling of a seismic anomaly. Production is obtained from Paleozoic carbonate reservoirs ranging in age from Ordovician to Mississippian. Cumulative production from the field, as of March 1982, is 1.2 million bbl. A mini-3D seismic survey was acquired in October 1981 to facilitate development drilling. The survey covered 2.4 mi2 (6.2 km2), encompassing the fields seven producing wells and two dry holes. The purpose of this survey was to provide an accurate image of the subsurface structure and delineate the extent of the producing formations. The areal coverage and improved subsurface imaging of the 3D survey provided a detailed view of the Medicine Lake anomaly. The seismic data reveals that the structure results from a local basement (Precambrian) high. Mapping of the Ordovician Winnipeg Formation revealed a domal structure covering approximately 0.6 mi2 (1.5 km2) with closure in excess of 180 ft (55 m). Although all producing wells are located on the Medicine Lake structure, stratigraphic variations within the reservoirs may localize production within structural closure. Porosity in several producing formations is diagenetic; prediction of reservoir trends from well data alone is difficult. Inversion and interactive modeling were used to study these stratigraphic variations. A correlation between relative acoustic impedance and porosity was established for several formations. Vertical and horizontal relative acoustic impedance sections were then employed to locate zones of possible porosity. This information, combined with the improved structural data, should aid in further development of the Medicine Lake field. End_of_Article - Last_Page 1353------------