Differential Pore Pressure Research Articles

Nigerian Case Study Explores Drilling Through Shales Below Depleted Sands

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 217195, “Drilling Through Shales Below Depleted Sands: Case Study of a Niger Delta HP/HT Gas Development Well,” by Stella Ogbodu, SPE, Bart Tichelaar, and Chibueze Amadi, SPE, Shell, et al. The paper has not been peer reviewed. _ Choosing the right casing depth is a key step in executing high-pressure/high-temperature (HP/HT) wells in the geologically complex Niger Delta. In the wells of Astra East Field, a casing shoe was set in a homogeneous shale of approximately 200 ft vertical thickness located between overlying heavily depleted sand and underlying overpressured sand. The question remained, however, as to what depth the casing shoe should be set to minimize the risk of kicks or wellbore instability and losses. To address this issue, wireline log data and drilling observations from other wells in the same field were analyzed and evaluated. Introduction The Astra East field was planned to provide feedstock gas. The field is in the Central Swamp depobelt of the Niger Delta and is 90 km west of Port Harcourt. More than 40 wells have been drilled in the field to date. A regional shale layer at approximately 11,400 ft subsea caps the regional top of overpressure in this field. This approximately 200-ft-thick shale layer vertically segments the field into two pressure regimes: a near-hydrostatic regime above it and an overpressured regime below. In other words, this shale layer acts as a regional pressure seal. Historically, oil and gas production has focused on the reservoirs above this regional seal, resulting in pressure depletion to pore-pressure values below hydrostatic in some of the reservoirs. Of the wells drilled in the field, only a handful penetrate the deeper overpressured section below the regional pressure seal and have faced various drilling challenges. Location and Geology of the Study Area The Astra East study interval is part of the Agbada formation, a paralic sequence of sands and shales up to 4 km thick in the central parts of the basin. Three lithostratigraphic subdivisions exist in the delta: the prodelta Akata shale formation, the paralic deltaic Agbada formation, and the continental Benin formation. These units represent an overall regressive, progradational stratigraphy. Depletion History and Drilling Performance The produced reservoir that sits immediately above the regional pressure seal is the C reservoir. Because hydrocarbon production from the C reservoir began more than 50 years ago, reservoir pressure gradient has depleted from approximately 5,150 psi before production to subhydrostatic. During the last 20 years, this depletion has remained relatively constant at approximately 1,660 psi below hydrostatic. The underlying, unproduced D reservoir is not depleted and is, in fact, at its virgin pressure, which is overpressured at approximately 3,035 psi above the hydrostatic pressure. This implies that the approximately 200-ft-thick intrareservoir shale between C and D reservoirs holds a large differential pore pressure of approximately 4,700 psi for the last 20 years. In fact, because the aquifer of the C reservoir was hydrostatic at virgin conditions, a large pressure differential was in place across this intrareservoir shale (between C and D reservoirs) even before production started.

Fault zone heterogeneities explain depth-dependent pattern and evolution of slow earthquakes in Cascadia

Slow earthquakes including tremor and slow-slip events are recent additions to the conventional earthquake family and have a close link to megathrust earthquakes. Slow earthquakes along the Cascadia subduction zone display a diverse behavior at different spatiotemporal scales and an intriguing increase of events frequency with depth. However, what causes such variability, especially the depth-dependent behavior is not well understood. Here we build on a heterogeneous asperities-in-matrix fault model that incorporates differential pore pressure in a rate-and-state friction framework to investigate the underlying processes of the observed episodic tremor and slow-slip (ETS) variability. We find that the variations of effective normal stress (pore pressure) is one important factor in controlling ETS behavior. Our model reproduces the full complexity of ETS patterns and the depth-frequency scaling that agree quantitatively well with observations, suggesting that fault zone heterogeneities can be one viable mechanism to explain a broad spectrum of transient fault behaviors.

Evaluation of the behaviour of the lateral boreholes in the Gorm chalk field

Open hole wellbores present cost efficient completion solution; however, instability of such boreholes in weak formations such as chalk in the course of production raises a concern. This paper presents a method for predicting the stability of the radial jet drilling laterals in the Danish Gorm chalk field. The method is based on four main parts: (1) rock mechanics testing in the triaxial cell; (2) the single lateral hole testing consisting of loading and fluid flow test; (3) utilizing CT imaging for identifying the damaged zone and its extension; and (4) numerical simulations, utilizing a chalk model that takes into account the post-peak softening and the rate dependency of the pore collapse stress. Simulation of bottom hole and reservoir pressure decline over a year by 17 MPa and 9 MPa, respectively, showed a small development of the plastic strain at the borehole wall. Further simulation up to four years with constant stress (creep) resulted in a change in the cross-sectional area of the borehole, where shear cracks developed at the wall and some distance away from the hole stress concentrations associated with pore collapse was observed. By the end of the simulation, the borehole was likely to change its geometry by removal of the plastified area. The single lateral hole test with flow carried out with differential pore pressure, drawdown, of about 2.5 MPa within 5 h and 1 h, respectively, provided insignificant permeability change during the flow test.

Collapse of Reacted Fracture Surface Decreases Permeability and Frictional Strength

AbstractGeochemical and geomechanical perturbations of the subsurface caused by the injection of fluids present risks of leakage and seismicity. This study investigated how acidic fluid flow affects hydraulic and frictional properties of fractures using experiments with 3.8‐cm‐long specimens of Eagle Ford shale, a laminated shale with carbonate‐rich strata. In low‐pressure flow cells, one set of samples was exposed to acidic brine and another set was exposed to neutral brine. X‐ray computed tomography and energy‐dispersive X‐ray spectroscopy revealed that samples exposed to acidic brine were calcite‐depleted and had developed a porous altered layer, while the other set showed no evidence of alteration. After reaction, samples were compressed and sheared in a triaxial cell that supplied normal stress and differential pore pressure at prescribed sliding velocities, independently measuring friction and permeability. During the initial compression, the porous altered layer collapsed into fine particles that filled the fracture. This effectively impeded flow and sealed the fracture, resulting in fracture permeability to decrease 1 to 2 orders of magnitude relative to the unaltered fractures. This is a favorable outcome in subsurface applications where the goal is to reduce leakage risks. However, during shear the reacted fracture had lower frictional strength because the fine‐grained particles in the collapsed layer prevented the formation of interlocking microasperities. Therefore, coupled geochemical and geomechanical processes that could favorably seal fractures could also increase the likelihood of induced seismicity. These findings have important implications for geological carbon sequestration, pressurized fluid energy storage, geothermal energy, and other subsurface technologies.

Rate‐and‐State Model Casts New Insight into Episodic Tremor and Slow‐slip Variability in Cascadia

AbstractAdvances in geodetic and seismic observations have led to the discovery of Episodic Tremor and Slow‐slip (ETS). ETS in Cascadia subduction zone occurs semiregularly and shows intriguing spatio‐temporal variability reportedly associated with frictional properties and stress conditions. Yet the origin of complex ETS behaviors remains largely unknown. Here we develop a laboratory‐based rate‐and‐state asperity‐in‐matrix subduction fault model, supported by geological observations of exhumed fault with heterogeneous frictional properties and pore pressure variation, to reproduce all ETS variability in good agreement with observations. Our results show that differential pore pressure plays a crucial role in affecting fault behaviors. Regions of asperities with decreased pore pressure tend to have increased tremor. Our study suggests that ETS variability can be used to probe otherwise enigmatic fault zone properties.

High sedimentation rates and thrust fault modulation: Insights from ocean drilling offshore the St. Elias Mountains, southern Alaska

The southern Alaskan margin offshore the St. Elias Mountains has experienced the highest recorded offshore sediment accumulation rates globally. Combined with high uplift rates, active convergence and extensive temperate glaciation, the margin provides a superb setting for evaluating competing influences of tectonic and surface processes on orogen development. We correlate results from Integrated Ocean Drilling Program (IODP) Expedition 341 Sites U1420 and U1421 with regional seismic data to determine the spatial and temporal evolution of the Pamplona Zone fold-thrust belt that forms the offshore St. Elias deformation front on the continental shelf. Our mapping shows that the pattern of active faulting changed from distributed across the shelf to localized away from the primary glacial depocenter over ∼300–780 kyrs, following an order-of-magnitude increase in sediment accumulation rates. Simple Coulomb stress calculations show that the suppression of faulting is partially controlled by the change in sediment accumulation rates which created a differential pore pressure regime between the underlying, faulted strata and the overlying, undeformed sediments.

The differential drying shrinkage effect on the concrete surface damage: Experimental and numerical study

This paper aims to study the relation between drying and the development of damage from concrete surface. For that reason, the experimental study aims to measure the internal relative humidity in four different locations during 500days. In parallel, total shrinkage is monitored for 250days in different positions of the specimen. Under drying conditions, low differential values are noticed. Higher porosity and pore radius in the specimen edge are observed. It can be explained by the effect of the drying on the cement hydration and the surface microcracks appearance. Based on the experimental results, a model which incorporates the main mechanisms for shrinkage is proposed. The difference between the free shrinkage provided by modeling and the measured strain profile allows determining the evolution of the internal stress by an iterative procedure. Finally, by defining the damaged depth in the specimen as the area where the tensile strength is exceeded, the evolution of this distance is deduced. The development of the damaged depth was very fast as long as the relative humidity was close to 100% in the specimen core with a high differential pore pressure between the center and the drying surface.

Absence of remote earthquake triggering within the Coso and Salton Sea geothermal production fields

AbstractGeothermal areas are long recognized to be susceptible to remote earthquake triggering, probably due to the high seismicity rates and presence of geothermal fluids. However, anthropogenic injection and extraction activity may alter the stress state and fluid flow within the geothermal fields. Here we examine the remote triggering phenomena in the Coso geothermal field and its surrounding areas to assess possible anthropogenic effects. We find that triggered earthquakes are absent within the geothermal field but occur in the surrounding areas. Similar observation is also found in the Salton Sea geothermal field. We hypothesize that continuous geothermal operation has eliminated any significant differential pore pressure between fractures inside the geothermal field through flushing geothermal precipitations and sediments out of clogged fractures. To test this hypothesis, we analyze the pore‐pressure‐driven earthquake swarms, and they are found to occur outside or on the periphery of the geothermal production field. Therefore, our results suggest that the geothermal operation has changed the subsurface fracture network, and differential pore pressure is the primary controlling factor of remote triggering in geothermal fields.

Evolution of Permeability during Fracturing Processes in Rocks under Conditions of Geological Storage of CO<sub>2</sub>

We investigated experimentally the change in permeability in three different types of soft rocks including mudstone, sandstone, and tuff from the Fureoi and Takinoue formations, which are representative of the CCS demonstration site at Tomakomai in Japan, and another type of mudstone from the Besho Fm. (B-M). Permeability during deformation, shear fracturing, and post-failure slipping is estimated from measured flow rate and differential pore pressure. In addition, the morphologies of the shear fracture zones were examined using the X-ray CT scanning technique. All the samples exhibit typical brittle-fracturing behaviors, except for the B-M sample which does not show a major shear fracture. During the fracturing process, the permeability increased by one to three orders of magnitude. Further changes in the post-failure slipping and stress relaxation regimes show strong dependence on the type of rocks. Observed changes in permeability can be interpreted as results of fracturing creation, shear zone smoothing, closure and reactivation of fractures under different stress regimes. [doi:10.2320/matertrans.M-M2015802]

Effect of Strain Hardening on Monotonic and Cyclic Loading Behavior of Plate-Fin Structures with Two Pore Pressures

We perform finite element homogenization (FEH) analysis to investigate the effect of strain hardening on the monotonic and cyclic loading behavior of plate-fin structures with two pore pressures. As a typical base metal of plate-fin structures, 316 stainless steel is considered and assumed to be the viscoplastic material that obeys the Ohno-Wang kinematic hardening rule. The plate-fin structures are assumed to be periodic and subjected to uniaxial monotonic and cyclic loadings in the stacking direction. A periodic unit cell is used for FEH analysis. Results are compared with those based on three special cases derived from Hill’s macrohomogeneity equation. It is found that the mean pore pressure entirely affect the homogenized viscoplastic behavior. It is further found that the differential pore pressure causes the remarkable accumulation of ratcheting strain in the periodic unit cell, although this internal ratcheting gives no effect on macroscopic relations, resulting in providing a closed hysteresis loop for the plate-fin structures.

Direct indication of pore-water advection from pore pressure measurements in Madeira Abyssal Plain sediments

The difference in pressure between the sea floor and a point in the underlying sediment is an indicator of the velocity of pore-water flow. This differential pore pressure has now been measured at five sites using a free-fall piezometer (pop-up pore pressure instrument, PUPPI) in a distal turbidite province of the Madeira Abyssal Plain. We found that at one of the five sites the pressure was slightly positive, at two others it was effectively zero, but at the remaining two sites the pore pressure was significantly negative (−450 and −120 Pa at 4 m depth). The implication is that at these last two sites water is moving downwards at rates of 3.1 and 0.8 mm yr−1 (calculated using laboratory-measured hydraulic conductivities from core samples). If this downward movement is part of a hydrogeological system which includes upward movement of pore water elsewhere, there are obvious implications for schemes to dispose of highly radioactive waste by burial in deep-sea sediments.

Closure to “Design of a Differential Pore Pressure Manometer and Its Time Lag Characteristics”

Closure to “Design of a Differential Pore Pressure Manometer and Its Time Lag Characteristics”

Design of a Differential Pore Pressure Manometer and Its Time Lag Characteristics

A new differential pore pressure manometer has been used to record pore pressure at the center of the specimen. Gibson’s theoretical analysis for time lag in base pore pressure is confirmed experimentally. Empirical equations are developed to predict time lag in pore-pressure measurement at the base as well as the center of the specimen. The technique of pore-pressure measurement by various kinds of probes is examined. Two sources of error—(1) choking of filter tip and (2) difference of ionic concentration in pore water and water in lead—are noted and remedial procedure suggested.