Macondo Pore Pressure and Stress Profile

The overburden stress is calculated by integrating the weight of the water column and the weight of the overlying sediment. We combine density log data from nearby wells in portions of the Macondo well where no density data were acquired. Logs are corrected to account for borehole washout and for the presence of hydrocarbons. Where no density data are available, a velocity-to-density transform is used31. If neither density nor velocity data are present, an exponential interpolation between density above and below the interval is used12.

Industry routinely measures pore pressure and takes fluid samples from relatively permeable formations with wireline tools (e.g. Modular Formation Dynamics TesterTM, MDT) and directly from the drill string (GeotapTM). At the Macondo well, BP recorded 21 pressures in four sandstones at the base of the well between 17,600 and 18,150 ft (5,364 and 5,532 m) (Fig. 2a, circles). 70 MDT pressures were recorded in nine sandstones between 8,900 and 12,500 ft (2,700 and 3,800 m) (Fig. 2a, squares) at the Texaco 252-1 well, located 1.27 miles (2.04 km) SW of the Macondo well. These MDT measurements are corrected to the Macondo well location assuming continuous stratigraphy parallel to the seafloor32.

We also constrain pore pressure from fluid influxes into the borehole (kicks) and elevated gas levels detected in the incoming drilling mud. Kicks and high gas occur when pore pressure exceeds hydraulic pressure from the drilling fluid in the exposed borehole. Six such events occurred during drilling operations (Figs 2, 3 and 5, open triangles). Using drilling information prior-to, during, and after an event, we estimate the location and pore pressure.

Drilling information includes the location of sandstones, length of exposed borehole, gas content of the incoming mud, surface mud weight, equivalent static density, equivalent circulating density, and shut-in drill pipe pressure. The equivalent mud weight is another way of expressing pressure using the average density of the drilling fluid from the drill floor to a location in the borehole. The equivalent static density is the downhole pressure expressed as an equivalent mud weight when the mud pumps are off and thus, there is no circulation. The equivalent circulating density is the downhole pressure expressed as equivalent mud weight as while the drilling fluids circulate. The circulating density is greater than the equivalent static density because of friction caused by fluid circulation.

The fracture pressure is the borehole pressure necessary to hydraulically fracture the formation. It is commonly close to the regional least principal stress but can be affected by stress perturbations due to the borehole geometry and the cohesive strength of the rock. The fracture pressure is constrained at four locations below the 9 7/8″ liner (Fig. 5). The downhole static and dynamic drilling pressures leading up to, during, and after each lost mud event are used to bracket the fracture pressure interpretations (Fig. 5, brown triangles). We define the upper bound of the fracture pressure with the equivalent circulating density when the losses began and the lower bound from the highest static or dynamic pressure at which the well is stable before or after the loss event (see ref.32 for detailed explanation). It is generally accepted that the in-situ stress of mudstone is higher than that of sandstone25, so the loss location is assumed to occur in the sandstone nearest to the bit at the time of the loss event. Fracture pressure is also constrained with the 9 7/8″ formation integrity test, FIT (Fig. 5, brown square). After drilling out of the cemented liner shoe, pressure on the exposed formation was increased to above overburden stress without experiencing fluid loss. This test result provides further evidence that the subsequent losses occurred deeper, in the M56 reservoir interval.

Mudstone Pore Pressure

Rapid deposition of this low permeability material is the primary source of overpressure in the Gulf of Mexico33. It is not practical to directly measure the pressure within these low permeability mudstones. Instead, mudstone pore pressure is commonly estimated from the compaction state (porosity) of the rock, which is typically measured by resistivity, density, or velocity34,35. In this approach, a correlation is established between one of these petrophysical proxies and the vertical effective stress, \({\sigma ^{\prime} }_{v}\). Once the correlation is established, then \({\sigma ^{\prime} }_{v}\) is determined given the observed property (e.g. velocity, density, resistivity). Once \({\sigma ^{\prime} }_{v}\) is determined, pore pressure, u, is easily determined if the overburden stress, σ v , is known (u = σ v − \({\sigma ^{\prime} }_{v}\)).

In deepwater Gulf of Mexico Neogene sediments, pore pressure is not accurately described by a single compaction curve. This is because deeper, hotter, and older mudstones have undergone more compaction than shallower mudstones at the same effective stress. Clay diagenesis is thought to be the primary cause of this behavior and the smectite-to-illite transformation (S/I) is considered the most significant36,37,38. More illitic material has a lower porosity at a given effective stress than a more smectitic material39,40. We follow ref.39 and assume:

$${\rm{\varphi }}-{{\rm{\varphi }}}_{{\rm{m}}}={{\rm{\varphi }}}_{0}{e}^{-B{{\rm{\sigma }}^{\prime} }_{{\rm{v}}}}$$ (1)

The left side of Eq. 1 is the total porosity, ϕ, less the pore volume that is filled by clay-bound water, ϕ m . The molecular structure of smectite has an easily hydratable interlayer, whereas illite does not41; thus the clay-bound water in the illite is less than that in the smectite (ϕ m , i < ϕ m , s ). The right side of Eq. 1 is a well-established trend for mudstone compaction (e.g. refs13,35) and here it describes intergranular porosity loss with effective stress. It is not well known whether ϕ 0 or B vary with the degree of the S/I transformation, so we assume that they are constant (ref.39)

We calibrate the model by determining the effective stress within mudstones adjacent to where pressure has been measured in sandstones. We assume that the overpressure, u*, in the mudstone equals u* measured in the nearby sandstone (e.g. ref.21), and use the mudstone pressure and overburden to calculate effective stress (u = σ v − \({\sigma ^{\prime} }_{v}\)). Next, we determine the mudstone porosity at each location from the velocity log after42:

$${\rm{\varphi }}=1-{(\frac{v}{{v}_{{\rm{ma}}}})}^{1/x}$$ (2)

where v ma is matrix velocity, v is the velocity log measurement, and x is an empirically derived acoustic formation factor exponent. We assume x = 2.19 and v ma = 14,909 ft/s (4,545 m/s) following precedent for Gulf of Mexico Neogene sediments21,35,42. The shallow locations with cooler in-situ temperatures have a higher porosity for a given effective stress than the deeper and warmer locations (Fig. 6). This contrast is most apparent at a vertical effective stresses equal to 1,500 psi (10 MPa) where the porosity, ϕ, in the shallow section is 9 porosity units greater (Fig. 6, green symbols) than the deeper section (Fig. 6, red symbols). We interpret that the deeper sediments have lost clay-bound water ϕ m as the smectite in the mudstone was converted to illite with burial.

Figure 6 Mudstone porosity vs. effective stress. Color-coded symbols denote in-situ temperature for each mudstone porosity-effective stress calibration point. The points are corrected for clay-bound water porosity (open symbols) and then are used to calibrate Eq. 1 (black line). Dashed lines show the porosity-effective stress relationships for different temperatures (color coded) and clay-bound water porosities, ϕ m . Measurements from the M56 (\({\sigma ^{\prime} }_{v}\) > 2,500 psi or 17 MPa) are corrected for hydrocarbon buoyancy. Porosity is estimated from velocity (Eq. 2). Full size image

We assume that porosity loss from clay-bound water release during the S/I transformation is linearly proportional to temperature, and that transformation begins at 70 °C and plateaus at 110 °C. This approximates the main phase of S/I transformation43,44,45 without additional constraints on depositional history and chemical composition46. We follow Lahann39 and assume ϕ m = 0.12 for smectitic mudstone and ϕ m = 0.03 for illitic mudstone. Based on these assumptions, the clay-bound water porosity is:

$${{\rm{\varphi }}}_{{\rm{m}}}=(1-\frac{{\rm{T}}-{{\rm{T}}}_{{\rm{s}}}}{{{\rm{T}}}_{{\rm{i}}}-{{\rm{T}}}_{{\rm{s}}}})({{\rm{\varphi }}}_{{\rm{m}},{\rm{s}}})+\frac{{\rm{T}}-{{\rm{T}}}_{{\rm{s}}}}{{{\rm{T}}}_{{\rm{i}}}-{{\rm{T}}}_{{\rm{s}}}}({{\rm{\varphi }}}_{{\rm{m}},{\rm{i}}})$$ (3)

where T is temperature, and T s and T i are the smectite (70 °C) and illite (110 °C) transformation boundary temperatures. We combine Eqs 2 and 3, and solve for ϕ − ϕ m for all the ϕ vs. \({\sigma ^{\prime} }_{v}\) points in Fig. 6. We then use least-squares regression to constrain Eq. 1 and find ϕ 0 = 0.22 and B = 2.9E−4 psi−1 (Fig. 6, black line).

Given B and ϕ 0 , Eq. 1 is then used to estimate mudstone pressure along the borehole (Fig. 2a, blue line) with ϕ m calculated from Eq. 2. To calculate mudstone velocity, we picked mudstones along the borehole at 30–40 ft (9–12 m) intervals and applied a 5-pick moving average to the corresponding compressional sonic log measurements. For each mudstone pick, we calculate ϕ from mudstone velocity (Eq. 2) and ϕ m from temperature (Eq. 3). ϕ and ϕ m are entered into Eq. 1, solving for \({\sigma ^{\prime} }_{v}\) and then u.

We apply this method (calibrated at Macondo) to estimate the mudstone pressure at 562-1 (Fig. 3). The close match between the estimated mudstone pressures and the measured sandstone pressures, independent of local calibration, supports the accuracy of our method within this region. Effective stresses at 562-1 are roughly 500-1,300 psi (3–9 MPa) higher than at Macondo (outside of the pressure regression). Mudstone sonic porosities are similar in both wells, but the temperature gradients are different. The Macondo well has an average temperature gradient of 28.4 °C/km versus 26.1 °C/km at 562-1. The lower temperature gradient and deeper water at 562-1 results in M56 temperatures that are nearly 20 °C lower than M56 temperatures at Macondo. The lower temperature indicates that the mudstone at 562-1 is more smectitic than the mudstone at Macondo for a given depth, so the sonic porosities transform to higher \({\sigma }_{v}^{\prime} \) (Fig. 6).

Aquifer Pressure

We determine the M56 aquifer overpressure at the Macondo well to be 3,386 psi (23.35 MPa), but it could be as high as 3,436 psi (23.69 MPa). At the Galapagos development, the M56 aquifer overpressure is tightly constrained to equal 3,433 psi (23.67 MPa). The overpressures are constrained with direct pressure measurements in the M56 sandstones at the Macondo well and three wells at the Galapagos development (Figs 1, 7). These wells are chosen because the pressure measurements were made before production at either location; thus, the measurements are interpreted to record the in-situ pressures unaffected by production or the Macondo release (Fig. 1, red circles and yellow stars). Many of the measurements were made within hydrocarbon-bearing sections. To determine the aquifer overpressure in such cases, the buoyant effect of the hydrocarbon column must be removed (e.g. ref.18). Specifically, the hydrocarbon pressure is projected down to the hydrocarbon-water contact (HWC) using the MDT-derived hydrocarbon density (Fig. 7). For each well at Macondo and Galapagos, we constrain the HWC, hydrocarbon-phase density, and water-phase density with log, MDT and seismic data. We then calculate aquifer overpressure at Macondo and Galapagos, taking into account pore-water density (u a * = u − ρ pw gz SS ).

Figure 7 Pressure vs. depth of M56 MDT measurements from four wells. Water-phase pressures for the Macondo and Galapagos structures are shown as blue dashed lines. A green dashed line denotes the M56 hydrocarbon gradient at Macondo. Solid horizontal lines locate observed and estimated hydrocarbon-water contacts. Full size image

At Macondo, we interpret that the 4-way closure of the M56 structure (Fig. 1b) was filled to its spill point. We interpret a structural crest at 17,720 ft (5401 m), a saddle at 18,375 (5601 m), and thus a column height of 655 ft (200 m) by depth-correcting BP’s predrill interpretation15. BP interpreted that the seismic amplitudes supported this filled-to-spill interpretation for the HWC15. We calculate the aquifer overpressure, u a *, to equal 3,386 psi (23.35 MPa) using a hydrocarbon gradient of 0.24 psi/ft (5.43 MPa/km) and a pore-water gradient of 0.465 psi/ft (10.52 MPa/km). It is possible that the structure was not filled to spill thus the HWC is shallower. LLOG-253-1 (Fig. 1, northernmost blue dot) provides the deepest hydrocarbon-bearing penetration of the M56 in the Macondo structure at 18,150 ft (5,532 m), which yields an upper bound to the aquifer overpressure of 3,436 psi (23.69 MPa)

The three Galapagos development wells (519-1, 519-2, and 562-1) (Fig. 1) constrain the aquifer pressure at this location to a single value (Fig. 7). At 519-1, two vertically stacked sandstone lobes comprise the M56. Each lobe shows a distinct HWC, but both share a u a * of 3,436 psi (23.69 MPa). 519-2 encountered only water in the M56, which yields u a * of 3,430 psi (23.65 MPa). We use these 519-2 MDT measurements to estimate the M56 pore water density of 0.465 psi/ft (10.52 MPa/km). 562-1 encountered hydrocarbon in the M56 and did not penetrate a HWC. An aquifer pressure calculation that assumes the HWC is just below the sandstone yields a u a * of 3,433 psi (23.67 MPa), which is nearly identical to those observed in the 519-1 and 519-2 wells. We use the average, 3,433 psi (23.67 MPa), to describe the aquifer overpressure at the Galapagos development.

Temperature Profiles

We determined the temperature profiles at Macondo and 562-1 using temperatures recorded during MDT pore fluid sampling (Fig. 8, open symbols). Temperatures between 113.3 and 113.7 °C were recorded at three MDT sample points in the Macondo well between 13,008 and 13,064 ft (3,965 and 3,982 m) below seafloor (Fig. 8, rectangles). At 562-1, four MDT sample points record temperatures between 93.5 and 98.4 °C for depths between 11,633 and 12,316 ft (3,545 and 3,754 m) below seafloor (Fig. 8, diamonds). BP’s temperature model for Macondo (Fig. 8, upper black line)8 is 3.8 °C higher than the average of the recorded temperatures in the M56 (Fig. 8, rectangle error bars). We assume this difference reflects a correction for borehole cooling. At Macondo, MDT measurements were acquired three days after drilling was completed, which is comparable to the four day gap at 562-1. Therefore, we apply the same 3.8 °C correction to the measurements at 562-1 (Fig. 8, diamond error bars). Our temperature model for 562-1 assumes a linear decrease from the corrected reservoir measurements to the seafloor (Fig. 8, lower black line). Seafloor water temperatures in deepwater Gulf of Mexico approach 4 °C for the water depths observed at Macondo and 562-1.