Borehole Fluid Research Articles

Modelling a shock wave source for seismic well survey

The shock wave seismic source has specific advantages for reservoir survey, particularly in the technique of vertical seismic profiling. This paper is concerned with the modelling of the characteristics of a shock wave device under conditions typical of its use as a surface seismic source. Such modelling necessitates numerical solution of the time-dependent conservation equations for axi-symmetric geometry under conditions where the fluid in the surface borehole may be air or water. A version of the piecewise parabolic method has been used to take account of the two-phase behaviour based on the formal properties of the well-known Tait equation of state for water. Results are presented which enable predictions to be made for use in the field of the pressure and velocity signatures of the shock wave source in terms of axial and radial profiles. Such information is significant in the assessment of the degrees of compressional- and shear-wave energies delivered by the source in vertical seismic profiling surveys of reservoirs.

Geochemistry of a sealed deep-sea borehole on the Cascadia Margin

The deep-sea borehole seal CORK was deployed for the first time on a modern accretionary prism during ODP Leg 146 to the Cascadia Margin. Ten months after the deployment the fluid flow and geochemistry of the borehole fluids was investigated during several dives by DSRV Alvin. The chemical analysis of the borehole fluids revealed methane concentrations of more than 3.5 mM, whereas oxygen and dissolved ions as Cl, NO 3, or PO 4 are still close to the ambient seawater composition. The exceedingly high methane content measured at the top of the sealed borehole and the observed degassing during the ascent of the submersible indicates that the sampled fluid was initially saturated or close to saturation with respect to CH 4. The hydrocarbons are characterized by C 1 C 2+ ratios of 170–200 and δ 13 C values of − 59.5 to − 62.4%o which indicates a considerable admixture of thermogenic hydrocarbon gases. The occurrence of methane of partly thermogenic origin demonstrates that CH 4 enters the sealed borehole in the lower, perforated section (94–178 mbsf) and accumulates at the top of the borehole. This suggests the occurrence of free gas within the encapsulated borehole. Considering the stability field of CH 4-hydrates, the formation of these ice-like structures may take place and potentially results in a clogging of the top of the borehole. Such precipitates could result in a decoupling of the top of the borehole from the hydraulic and geochemical regime of the accretionary complex, an important aspect for future plans of CORK deployments.

Shear slowness measurement by dispersive processing of the borehole flexural mode

In sonic well logging, the flexural mode generated by a dipole transducer allows measurement of shear slowness in slow formations, that is, formations in which the shear slowness is slower than the borehole fluid speed. The flexural mode is a dispersive borehole mode, but many current processing methods do not account for this dispersion. The approximate semblance equation is derived to explain what happens when a dispersive wave is processed nondispersively. The processing described here is a dispersive analog of the slowness‐time coherence (STC) processing used commonly for array sonic waveforms. It back‐propagates waveforms according to model dispersion curves and calculates semblance. It is a specialized version of the maximum likelihood (ML) and least‐mean‐squared‐error (LMSE) estimator for formation shear slowness. The dispersive analog of STC is too slow for depth‐by‐depth logging but may be a helpful analytic tool. A high‐speed variant, dispersive STC (DSTC), eliminates this difficulty by a new window‐positioning technique and can be faster than STC. DSTC results based on data from a real‐axis integration model show that the technique measures the formation shear slowness with small bias because of windowing and other arrivals. Results are presented from two wells logged with a dipole sonic tool. A log from a fast formation shows that shear slownesses measured from the flexural mode by DSTC agree with monopole results. A log from a slow formation is also presented.

Supersaline and metal-rich brine obtained from the Quaternary Kakkonda Granite by NEDO WD-1a in the Kakkonda geothermal field, Japan

A supersaline, metal-rich brine (ca. 40 weight% total chloride salt) was extracted from 3708 m depth of well WD-1a in the Kakkonda Granite by reverse circulation after a standing time about 196 hours. The estimated borehole temperature exceeds 500 °C near the bottom. Tritium content and the relationship between δD and δ18O show that the injected borehole fluid (river water) evidently mixed with an isotopically heavy and ancient fluid. The phase analysis showed that a gas phase separated from a brine and that a brine concentrated in the borehole as the borehole temperature recovered after cooling by drilling fluids. We think the original fluid was trapped in the Kakkonda Granite and mixed with the borehole fluid through fine fractures induced by thermal stress.

Complementary functions reveal data hidden in your logs

Abstract Many logging tools make multiple measurements of the same type that have more than one depth of penetration. Common examples are Compensated Density, Compensated Neutron and Array Induction logs. The purpose of the compensation is to reduce or remove the effects of a disturbance that distort the true measurement. Examples of this disturbance are the borehole size, mudcake and salinity. A general technique can be derived based on a theory of Linear Perturbation which requires no prior knowledge of the nature of the perturbation, the only requirement being that it is approximately locally linear. Various interpretations can be made of the general equation depending on the particular circumstances. The technique also produces a Complementary Parameter associated with the degree of correction. This parameter is usually discarded or paid scant regard, but can often be of some significant value and exposes surprising information. A number of examples can be used to illustrate these techniques, showing that they have wide applicability in situations ranging from difficult logging conditions (e.g. density through casing) to the apparently routine, where unusual and unexpected borehole fluids are revealed from neutron logs.

Hydrophone VSP imaging at a shallow site

We evaluated the capabilities of vertical seismic profiling (VSP) for imaging the complex heterogeneous unconsolidated sedimentary structures at a shallow site. We deployed a 24‐level hydrophone array with 0.5-m level spacing down a preexisting poly vinyl chloride (PVC) cased well. Data acquisition time was quick. Only 15 multioffset shot points using a hammer‐on‐plate source were needed to acquire reflection data between the water table at 3 m and the bedrock at 35 m to produce a depth section image. This image extended 9 m from the receiver well, yielding resolutions between fresh‐water‐bearing sands and impermeable muds and clays of better than 1 m. Depth accuracy of the image was confirmed by good correlation with cone penetrometer logs. We used conventional wavefield separation and VSP-CDP mapping techniques to image the data. Tube waves, created by seismic arrivals at cross‐sectional area changes in the borehole fluid column, were the primary source of coherent noise in the data. The tube‐wave arrival structure was complicated by the hydrophone array, which generated and scattered tube waves at each hydrophone pod. To combat the tube wave interference, we inserted closed‐cell‐foam baffles between elements. The baffles attenuated and slowed the tube waves, and reduced generation and scattering. A comparison between unbaffled and baffled VSP data showed that baffling increased the maximum useful frequency from 300 Hz to over 900 Hz. By contrast, surface shot data recorded at the same site, using buried 40-Hz vertical geophones, exhibited useful frequencies of less than 250 Hz. In addition, coherent noise in surface shot records caused by air waves and first arrivals made it very difficult to identify shallow reflections above 25 m. Reflections from depths as shallow as 10 m were easy to identify in the baffled VSP data.

Changes in attenuation with depth in an ocean carbonate section: Ocean Drilling Program sites 806 and 807, Ontong Java Plateau

Changes of in situ seismic attenuation with depth are estimated from full waveform acoustic logs in a soft formation (rock with shear velocity less than borehole fluid velocity). The relative attenuation Qp−1 is computed from the variation with depth of the P wave amplitude spectrum of a single source receiver pair. The method is applicable when unknown source and receiver responses of the sonic tool make other methods difficult to apply. Tests with synthetic data generated by a full waveform method verify that in a soft formation the decay of the P wave amplitude spectrum depends mainly on attenuation. The method is applied to Ocean Drilling Program data from holes 806B and 807A on the Ontong Java Plateau in the western Pacific Ocean. Four attenuation logs were computed independently using data from four source receiver pairs. Agreement of the four logs at each site and agreement of results from the two sites suggest that the method is robust and practical. The attenuation logs show a maximum attenuation between 300 and 500 m below the seafloor. They agree well with compilations of high‐frequency attenuation versus porosity and frequency in marine sediments but are somewhat greater than results from seismic experiments, possibly owing to the presence of sedimentary microbeds and the three‐decade difference between the seismic and sonic frequency bands.

Sensitivity and inversion of borehole flexural dispersions for formation parameters

Propagation characteristics of borehole flexural waves are functions of five borehole, fluid and formation parameters. Uncertainties in any of these model parameters may introduce errors in the estimate of formation shear-wave speed. Other sources of error in the estimate may be caused by deviations from the assumed circular borehole cross-section or heterogeneity in the material properties of formation in the vicinity of a borehole from the usually assumed homogeneous properties. The influence of uncertainties in model parameters on borehole flexural dispersion has been calculated from a general perturbation model based on Hamilton's principle. A sensitivity analysis of the flexural dispersion to small variations in the model parameters shows that the formation shear speed has by far the most dominant influence in a slow formation. In contrast, the flexural dispersion in a fast formation is significantly influenced by three of the five model parameters: the formation shear speed, the borehole fluid compressional speed and the borehole diameter. The frequency dependence of these sensitivity functions indicates that the inversion of flexural dispersion for formation shear speed is optimal in the range 2–4 kHz for a borehole of diameter 25.4 cm. The range of validity of the perturbation model has been estimated by comparing results of concentric annuli of different thicknesses, and shear-wave speeds different from that of the formation with those from exact numerical solutions from a modal search program. Generally, the perturbation results for altered annuli of thicknesses up to 15 cm are accurate to within 1 per cent for a shear-wave speed 10 per cent lower than that of the formation. This difference between the perturbation and exact results increases to approximately 1 to 3 per cent for a shear-wave speed 20 per cent lower than that of the formation.

The Monte Carlo Multiply Scattered Components (MCMSC) approach for natural gamma-ray log interpretation

A new approach called the Monte Carlo Multiply Scattered Component (MCMSC) approach has been developed for natural gamma-ray logging tool interpretation. It consists of first determining individual scatter component library gamma-ray spectra for each of the three sources of natural gamma radiation (/sup 40/K, U, and Th) for a reference sample by Monte Carlo simulation. Then these scatter component library spectra, normalized to unit area, are used to obtain coefficients by the library least-squares (LLS) method with the natural radioisotope library spectra for a range of sample parameters such as borehole fluid density and composition as well as formation density and composition. Finally, empirical relationships are developed between the individual scatter component library coefficients and the sample parameters. It is found that the normalized scatter component libraries are insensitive to the formation lithology, and the range of allowable parameter variations covers most of the sedimentary formations. This approach allows one to construct the appropriate natural radioisotope libraries for /sup 40/K, uranium, and thorium for a sample with known characteristics in a very efficient and accurate manner. This, in turn, allows the calculation (again by the LLS method) of amounts of the three sources of natural gamma rays for any sample spectrum.

Influence of an Invaded Zone on a Multiprobe Formation Tester

Summary When an oil or gas well is being drilled, some of the borehole fluid (mud filtrate) leaks into the formation, displacing the native reservoir fluid. This creates an invaded zone around the wellbore. The invading fluid will generally have a mobility and compressibility that differ from the formation fluid. The presence of the invaded zone will affect the pressure transients measured by a formation tester. In this paper, a model that includes an invaded zone is presented together with an analysis of its effect at each of the probes of a multiple probe formation tester. The results show that the properties of the invaded zone dominate the pressure transient measured at the sink probe, strongly influence the transient at the horizontal probe, and only modestly affect the vertical probe transient. A suggested modification to the current interpretation procedure (which does not consider the invaded zone) for determining the horizontal and vertical mobility is also presented. The application of this modified procedure is discussed using a model example.

Pressure in a fluid‐filled borehole caused by a seismic source in stratified media

A method for numerically simulating hydrophone vertical seismic profiles (VSP) and crosswell data measured in a fluid‐filled borehole (either open or cased) embedded in stratified media is presented. The method makes use of both the borehole coupling theory and the global matrix formulation for computing synthetic seismograms in a stratified medium. The global matrix formulation is used to calculate the stress field at the borehole location. Borehole coupling theory is then employed to obtain the pressure in the borehole fluid. Comparisons with exact solutions for an open borehole in a homogeneous and unbounded formation show that this method is accurate for frequencies below 2 kHz. This method is used to model the Kent Cliffs hydrophone VSP data, where good agreement between the numerical simulations and the field measurements has been found, in both traveltimes and rms amplitudes of the direct P‐wave. Examples show that this method is efficient and accurate, and can be applied to model VSP and crosswell experiments using an array of hydrophones.

Stoneley and flexural modes in pressurized boreholes

The propagation of Stoneley and flexural waves in a fluid‐filled borehole is adequately described by the linear equations of elasticity. However, when the borehole fluid is pressurized either due to the hydrostatic head at a given depth or with the aid of packers at the wellhead, both the fluid and the surrounding formation are subjected to biasing stresses. Under this situation, wave propagation along the borehole is described by the equations of motion for small dynamic fields superposed on a bias. The resulting formulation allows us to study the influence of a change in the fluid pressure on the Stoneley and flexural mode dispersion curves. Since the biasing stresses in the surrounding formation exhibit a radial decay away from the borehole, it is expedient to employ a perturbation technique to calculate changes in the borehole Stoneley and flexural wave dispersion curves as a function of hydrostatic pressure change in the fluid. A key advantage of this perturbation technique is that it separates contributions of the acoustoelastic effect due to the borehole fluid and that due to the formation. Insofar as the fluid nonlinear properties at a given pressure and temperature are known, the model provides a procedure for estimating the acoustoelastic coefficient of the formation for the borehole Stoneley and flexural wave velocities for a given change in the fluid pressure. The formation acoustoelastic coefficient can be expressed as a fractional change in the acoustic wave velocity caused by a unit change in the borehole pressure. Computational results show that acoustoelastic coefficients for the Stoneley and flexural waves are larger for formations with higher degree of nonlinearity which is typically associated with poorly consolidated rocks.

Compressive and tensile failure of inclined well bores and determination of in situ stress and rock strength

In this paper we investigate the occurrence of compressive and tensile failures of arbitrarily inclined well bores under a wide variety of stress conditions. The principal assumptions in this analysis are that the rock is isotropic and that it deforms elastically to the point of failure. As has been shown by previous investigators, for a given stress state and well bore orientation, it is straightforward to predict the orientation of the failures around the well bore as well as whether failure is likely to occur depending on such parameters as rock strength and borehole fluid pressure. However, as the stress state is almost never known in situ, we demonstrate how observations of compressive and tensile wall failures in inclined holes can be used to constrain in situ stress orientations and magnitudes if there are independent data on the magnitude of the least principal stress from either leak‐off or microfrac tests and on the formation pore pressure. We further demonstrate how once the stress state is determined, it is possible to assess both an upper bound on the effective in situ rock strength and the degree to which increasing the borehole fluid pressure (or mud weight) can reduce the likelihood of borehole failure. Through application of this methodology to an inclined well bore in an area of complex faulting in the Gulf of Mexico, we illustrate how it is possible to utilize observations of borehole failures to determine the magnitude and orientation of the stress tensor in areas such as offshore sedimentary basins where drilling inclined well bores is quite common.

Distortion in resistivity logging at shallow depth

Owing to the proximity of an insulating ground surface, normal resistivity logging at shallow depths (less than 30 m) can yield an apparent resistivity that exceeds 200% of the formation resistivity for a homogeneous medium. The distortion is more acute for long‐normal than for short‐normal logging. Three examples from a landfill site in southern California are presented to show such distortion. The patterns of distortion are similar for logging devices consisting of either two point‐source electrodes or one point‐source and one finite length, line‐source electrode. The former electrode array is a generally accepted approximation of the latter. However, the simulated apparent resistivity for the line‐source array is greater than that for the point‐source array at any given depth. A resistivity contrast between the formation and the borehole fluid can shift the magnitude of the background apparent resistivity but does not significantly alter the pattern of distortion. The magnitude of the distortion can be reduced by placing the reference‐ground potential electrode at a radial distance that is about equal to the spacing between the downhole upper potential electrode and the upper current electrode. It can also be removed by including the radial distance in an array‐dependent geometric factor that accounts for the resistivity of the borehole fluid and the proximity of the logging device to the ground surface.

In-Situ Stress and Rock Strength in the GBRN/DOE Pathfinder Well, South Eugene Island, Gulf of Mexico

The authors present a relatively simple technique to constrain in-situ stress and effective rock strength from observations of wellbore failure in inclined wells. Application of this technique in the Global Basins Research Network (GBRN)/DOE Pathfinder well demonstrated that (1) the azimuth of S{sub h min} is {approx} N42{degree}E, perpendicular to a major growth fault penetrated by the well; (2) the magnitude of S{sub H max} is relatively close to the vertical stress; and (3) the effective in-situ compressive rock strength is 3,500 to 4,000 psi. They show that once they have estimated in-situ stress and rock strength, it is possible to compute the mud pressure required to inhibit failure for wells of any azimuth and inclination. Finally, they show how it is possible to estimate the magnitudes of both S{sub h min} and S{sub H max} in cases where independent knowledge of stress orientation is available (for example, from wellbore breakouts in nearby vertical boreholes).

The Hydraulic Effects of Lost Circulation and the Implications for Contaminant Migration During Drilling

AbstractThe impact of lost circulation during rotary drilling near an existing monitoring well cluster was evaluated by periodic measurements of water levels and contaminant concentrations at the well cluster. Due to regulatory concerns, changes in water levels or VOC concentration in the well cluster during drilling would trigger monitoring well redevelopment. The borehole was drilled approximately 30 feet northeast of four nested monitoring wells that screen Devonian and Silurian carbonate bedrock at depths of 15, 60, 130, and 190 feet. Following complete circulation loss at depths of 177 and 1 S3 feet in the borehole, a rapid decrease in water levels was observed in the upper three monitoring wells. The water level in the well that was screened through the lost circulation zones increased slightly.Decreasing water levels in formations located above the point of circulation loss appear to occur in response to a sudden decrease in borehole fluid pressure caused by the flow of drilling fluid into the formation. The relative contribution of contaminated formation water lo the borehole can be estimated by using the time‐drawdown relationship and estimates of transmissivity. At the point of circulation loss, significant dilution of contaminant concentrations occurs from the loss of drilling fluid into the contaminated zone. Contaminated formation water entering the borehole during periods of complete lost circulation may mobilize contaminants from upper lo lower formations. Lost circulation into a formation would be signaled by a water level increase in monitoring wells. The wells would subsequently require development to remove the volume of fluid lost to the formation, including both drilling fluid and contaminated formation water. Monitoring wells exhibiting declining water levels following lost circulation would not require development since drilling water has not entered the zones screened by these wells.

Conductive heat flow and thermally induced fluid flow around a well bore in a poroelastic medium

Transient analytical solutions for temperature and pore pressure changes near a circular borehole under instantaneous temperature and fluid pressure changes inside the borehole are presented. The solutions couple conductive heat transfer with Darcy fluid flow, and a borehole under a nonhydrostatic far‐field stress state is simulated. The heat conduction equation is decoupled from the coupled system of isothermal governing equations, and the complete solution is obtained by superimposing this decoupled solution on the isothermal one. The solution is therefore applicable to low‐permeability media, where heat transfer is dominated by conduction only. Both cold and warm injection processes are studied, and the applications to hydraulic fracture initiation and thermally induced fluid flow are discussed. Taking Westerly granite as an example, it is concluded that the maximum thermally induced pore pressure inside the rock formation can be 30% higher than the isothermal pore pressure, with a borehole temperature and fluid pressure change ratio (ΔT/Δp) = 1°C/MPa. It is emphasized that the thermally induced pore pressure change can be significant inside a low‐permeability porous medium, and a coupled solution must be obtained to address the mechanical, hydraulic, and thermal responses appropriately.

Generation of Hydrogen Gas as a Result of Drilling Within the Saturated Zone

AbstractHydrogen gas was discovered within the steel casing above standing water in a percussion‐drilled borehole on the Hanlord Site in south‐central Washington state. In situ measurements of the borehole fluids indicated anoxic, low‐Eh (<‐400 mV) conditions. Ground water sampled from adjacent wells in the same formation indicated that the ground water was oxygenated. H2 was generated during percussion drilling, due to the decomposition of borehole waters as a result of aqueous reactions with drilled sediment and steel from the drilling tools or casing. The generation of H2 within percussion‐drilled boreholes that extend below the water table may be more common than previously realized. The ambient concentration of H2 produced during drilling was limited by microbial activity within the casing‐resident fluids. H2 was generated abiotically in the laboratory, whereby sterilized borehole slurry samples produced 100 times more H2 than unsterilizcd samples. It appears that H2 is metabolized by microorganisms and concentrations might be significantly greater if not for microbial metabolism.

Increased Thermal‐Pulse Flowmeter Resolution by Adding a Packer and Computer

AbstractMeasurement accuracy was increased by nearly one order of magnitude by outfitting the thermal‐pulse flowmeter (TFM) with an inflatable packer. To accurately measure slow water velocities in boreholes greater than 15 cm diameter, it is necessary to divert borehole fluids through the TFM by inflating a packer. During calibration it was noted that the TFM's accuracy decreased as the borehole diameter increased. With Lhe packer inflated the TFM has a useful flow measurement range of 0.08 to 15 L/min (with flow velocities of 0.24 ± 0.012 cm/inin to 45.7 ± 0.61 cm/min, respectively, in 20‐cm‐diameter pipe), compared to 0.8 to 57 L/min for a packcrless TFM. A computer interlace was added to the TFM to provide a real‐time graphical display of the differential voltage output from the TFM, a running mean and standard deviation of the pulse‐response time, and a mean flow rate and velocity based on calibration curve fits.

Deconvolving Ocean Drilling Program temperature logging tool data to improve borehole temperature estimates: Chile Triple Junction

We present a technique for correcting borehole fluid temperature observations made by the Ocean Drilling Program (ODP) with the Lamont temperature logging tool (TLT), for the effects of the slow temperature response of one of its sensors. TLT data have been recorded in many ODP boreholes, but, perhaps partly because of tool response effects, the data have only rarely been used. It has been shown that a continuous temperature log is the convolution of a tool response function and the temperature history experienced by the tool. We use temperature data from ODP Leg 141 to estimate the tool response function of the TLT. We then use Wiener filter theory to design a decon volution operator to remove the effect of the tool response from the recorded data. We apply the deconvolution operator to the data from Leg 141, assess the effectiveness of the deconvolution technique, and extrapolate the resulting borehole fluid temperatures to estimate the equilibrium geotherm at the two sites considered. The geothermal gradient in the accretionary wedge near the Chile Triple Junction increases with depth. This suggests that the thermal environment is not steady state, that fluid flow is transporting heat, or, most likely, both. The average heat flow in the accretionary wedge near the Chile Triple Junction is higher over the subducting Chile Ridge axis than over subducting young oceanic crust near the ridge axis.