Why Stresses from Reflection Seismic are Accurate 25 May 2016

All well objectives require drilling with or against Earth stresses. SHmax magnitude (SHM) from well data are post-drill, estimated, and are only available at spot points. It is crucial for well design to have accurate SHM and SH directions (SHD) available, pre-drill, in 3D. This can only occur using close-spaced 2D or 3D reflection seismic.

Therefore the question arises, how do we know the 3D stress data are accurate? The basis is that the shapes of the deformed seismic horizons were determined by the stresses causing the deformation, therefore it should be possible to determine those Earth stresses if we can accurately describe the deformation. For example, the top blue anticline in Figure 1 is shown on an isochore or thickness of the sediments formed between the map of an horizon below the seafloor horizon and the map of the base of the Pliocene 5.8 million years ago. The stresses that formed the deformed isochore are a function of the weight of the sediments, SV and the Anderson maximum and minimum horizontal stress components SH and Sh, respectively. In this instance the patented DrillAssure software calculated a low value for SHM = SH/SV = 1.0 for the anticline, surrounded by a lower 0.875.


Fig. 1. Structure Map Validation. (Click to view enlarged image)

During initiation of a basin some 4 km to 5 km below the fourth isochore, the basement surface imparts the Earth stresses to the sediment isopach immediately above it together with the impression of the developing faults. All mapped isochores of the deformation can be converted to quantified stress maps using DrillAssure using just the load SV and the Anderson stress states. As described in Figure 1, SHM is progressively muted up-section due to decreasing lithification resulting from increasing distance from the hinge-like, upward bending, upper crustal stress source. The interpretation quality of applied stress transformed from a deformed horizon shape is solely dependent on seismic data quality, reflection horizon/fault picking and best knowledge of SV.

On increasing burial the load increases as does SH so that SH/SV (=SHM) increases as described in Figure 1. In the case of a simple rift or margin sag basin the uninterrupted progression of blue (low Anderson stress states at the surface) to green and pink (higher Anderson stress states at depth) is indicative of normal increasing load and SHM development with upper crustal compression.


Fig. 2. Quantitative Pressure-depth graphs. Click to view enlarged image

The isochores penetrated by the vertical well in Figure 2 record the Anderson Stress States and have been used to construct a Quantified Pressure-Depth Graph (PDG) at left. Knowing the rock load SV is close to linear, varying numerical values for SH and Sh were calculated by DrillAssure and plotted as composite black and red lines. Due to the pulsed nature of the anticlinal growth and pulsed pore pressure (PP) build-up followed by seal fracturing, it is possible to calculate with high accuracy (slightly below the regional) the blue PP gradient relative to the red fracture gradient (Sh/SV) on the left of the PDG.

Why plan complex horizontal wells which you may want to fracture stimulate (parallel with SHD) using sparse, post-drill 1 and 2-dimensional extrapolated well-bore stress measurements, when you can have pre-drill, 3D Earth stresses generated from the interpreted close-spaced 2D or 3D reflection seismic you have already paid for? Stress check your prospect as you map and place the well stress advantageously.

It is possible to plan a well anywhere from seismic with only a knowledge of the approximate age and load of the rocks and whether they are likely to be soft (generally Tertiary-Jurassic) or hard (pre Jurassic); no offset well measurements are required.