This paper shows a comparison of dual-array microseismic maps with single-well maps for horizontal wells in the Barnett shale. Results from two test cases showing gel and water fracturing maps are given and compared with initial production. Dual-array mapping provides for a much larger areal coverage and increased accuracy when accurate bottomhole locations and velocity structure are available, but do have trade-offs that need to be considered.

Passive seismic imaging is a quickly growing technology to map fracture growth during hydraulic fracture stimulations, map active fracture networks, monitor well failures and track injection of fluid and steam. In this paper a number of issues are presented which should be considered when applying some potential applications are described, along with potential pitfalls that a potential user should consider.

This paper describes the development and capabilities of a novel and unique tool that interfaces a hydraulic fracture model and a reservoir simulator. This new tool is another step in improving both the efficiency and consistency of connecting hydraulic fracture engineering and reservoir engineering.

The typical way to model hydraulically fractured wells in 3D reservoir simulators is to approximate the fracture behavior with a modified skin or Productivity Index (PI). Neither method captures all the important physics of flow into and through the fracture. This becomes even more critical in cases of multiphase flow and multi-layered reservoirs. Modeling the cleanup phase following hydraulic fracture treatments can be very important in tight gas reservoirs, and this also requires a more detailed simulation of the fracture. Realistic modeling of horizontal wells with multiple hydraulic fractures is another capability that is needed in the industry. This capability requires more than an approximate description of the fracture(s) in the reservoir simulation model.

The Athel exploration play in the South of Oman contains a huge oil reserve, but in rather unusual reservoirs with very low permeability. Two discovered fields, Al Noor and Al Shomou carry a total oil in-place in excess of 2 billion barrels. These reservoirs contain thick silicilyte slabs that are encased in salt and shale. Due to low permeability, effective well stimulation plays a crucial role in economic field development.

There was evidence that silicilyte cores contained salt and it was believed that removing the salt with a simple fresh water soak could increase the permeability. Subsequently, a series of fresh water injection experiments was conducted in an Athel well, in 1995-97. In total, more than 16000 m3 (100,000 bbl) were injected. The aim was to wash the salt from the matrix, increasing the near wellbore permeability and hence, improving productivity. The well was indeed stimulated by these treatments, but the improved productivity did not last for very long.

Later, we suspected that the fresh water injection could have resulted in fracture propagation, or other production improvement mechanisms. A fresh look at the injection and production test data was conducted in 1998, with the objective of assessing the applicability of water-frac treatments. This could potentially eliminate or significantly reduce the need for proppant in fracture treatments.

The study showed that the production improvement was not due to matrix salt washing. In fact, hydraulic fractures were propagated. But the production response after the final and largest volume fresh water injection was complex and difficult to model. Apparently, in addition to fracture propagation, a large cavity was also dissolved in the overlying salt layer. The back production of salt saturated water, and increasing storage coefficient as derived from successive well test analysis could be related to the creation of such a cavity. In addition, the rock might have dilated, contributing to the short lived, but significant production increase. The study concluded that so-called “water-frac” treatments would need proppant to maintain fracture conductivity in the Athel.

This paper presents a field case of a novel stimulation method in tight reservoir rock. The methodology of integrating well tests analyses, production logs and fracture geometry simulations to obtain a comprehensive reservoir stimulation model is outlined. The production mechanisms induced by the fresh water injection and the cause of the rapid productivity decline is described.

In scaled laboratory tests, we perform acoustic measurements in a time-lapse sequence to separate the fracture response from the background signal. Using both compressional waves and shear waves (that are very sensitive to fluid filled fractures) we can not only detect the hydraulic fracture, but also characterize its shape and geometry during its growth. We show the application of the technique to propagation, flowback tests and re-opening of hydraulic fractures.

During fracture growth the acoustic waves excite diffractions at the tip of the fracture. We use the direct compressional and shear diffractions to locate the position of the tip of the fracture. The ultrasonic data show that the diffractions are most strongly excited at the fluid front of the hydraulic fracture. In some cases, a much weaker precursor diffraction can be detected which is interpreted as the dry tip of the fracture. Depending on the acquisition geometry, we detect many events related to surface waves propagating along the fracture. Also, we observe that shear waves detect the migration of the fluid front during re-opening of a pre-existing hydraulic fracture, in contrast with the compressional waves which are insensitive to the fluid front during re-opening.

Observation of transmission signals through the fracture yields a very accurate measurement of the fracture width. In this way we have obtained the full fracture width profile during propagation and closure. Flow back resulted in closing the fracture at the wellbore, while it remained open farther from the wellbore.

The surface roughness of hydraulic fractures reflects the fracturing process at the tip. We did an experimental study of hydraulic fracture propagation, in which a characteristic roughness pattern was observed. This roughness developed without shear- or torsional loading in the plane of the penny-shaped fracture. The roughness showed to be determined by material properties and the externally applied stresses on the sample, which represent the effective in-situ stresses. We quantified the roughness, and showed that it correlated well with a measure of the plastic zone size around the fracture tip. New explanations for fracture surface roughness that develops under mode I loading conditions are presented.

In the main KTB-well (Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland), we pumped 210 m3 of brine to make a hydraulic fracturing stress measurement and to induce seismic events. We observed a peculiar pressure behavior: a clear breakdown occurred at low flow rate, but at higher flow rate the pressure kept rising until we reached the pressure limit. During the later injection stages, a large number of triggered seismic events occurred. The events mostly came from faults at a depth of 8.6 and 8.9 km, whereas the fluid was injected at 9.1 km depth. A few events came from faults as shallow as 7.2 km. Careful analysis of the pressure record showed that no tensile fracture was created. The seismic events indicated that the injected fluid was most probably going into pre-existing shear faults.

This paper describes laboratory tests and numerical simulation of hydraulic fracture behaviour during propagation and closure. The tests were carried out on cement and sandstone blocks. Pressure and width were measured at the borehole and active acoustic monitoring was used to determine the fracture shape. The numerical results predicted the laboratory results during fracture propagation and closure.

Current practice, for in situ calibration of closure pressure and fluid loss, is based on simplifying assumptions for shut-in behaviour. From the experiments and simulations, the paper provides recommendations for ensuring correct interpretation of field data.

A hydraulic fracture treatment in a cased-perforated well may result in the initiation of inclined starter fractures from each perforation. We performed scaled model tests and observed that link-up of starter fractures was affected by the confining stress difference and the pressure in the fracture during a treatment. A large stress difference in the plane perpendicular to the perforations prevents starter fractures from linkage. Also, fracture linkage is affected by the cementation of the well. Fluid flow in the annulus between borehole and casing may results in better linkage, but the fracture may grow through the annulus and avoid the perforations.

The estimation of the reservoir permeability, a variable of great importance in hydraulic fracture design, is frequently inaccessible because candidate wells either do not flow or a pretreatment pressure transient test would be inordinately long (and expensive). A methodology for the interpretation of a fracture calibration treatment has been developed that allows the calculation of the permeability in addition to the traditional results of the "leakoff coefficient", closure pressure and fluid efficiency. Here, gas reservoir field cases are presented and interpreted with this method. A log-log diagnostic plot of the rate-normalized pressure and its derivative was employed to account for a multi-rate injection (fluid leakoff) and it is used to identify the transient reservoir response during fracture closing. The reservoir permeability can then be estimated from a specialized plot. This procedure is verified by a pressure match.

For data from fracture calibration treatments a methodology is introduced for the determination of the reservoir permeability as an addition to the usual variables of leakoff coefficient and closure pressure. The technique derives from the solution of the diffusivity equation for a well with an infinite-conductivity vertical fracture (as a multi-rate injection) with a superimposed varying filtercake skin effect. The transient reservoir response, which is found to be the essential feature, is then decoupled and used to determine the reservoir permeability.

A potentially important component of the total pressure drop from a fracture into a formation during hydraulic fracture stimulation is that across the polymer invaded zone. Following experimental work with long cores (up to 60 cm-long) and with permeabilities up to 5 md it was determined that the polymer invasion for a zirconate- crosslinked HPG-system has not particular significance and is thus negligible compared to the very dominant filtercake. The observed early- and late-time filtration phenomena were described using a numerical cake layer model and verified by experimental measurements. It is shown that the early-time "hyperbola" is not a result of polymer invasion but is caused by a changing pressure differential across the filtercake at the beginning of the filtration experiment.

Static leakoff experiments with filterpaper, using borate- and zirconate-crosslinked hydroxypropylguar (HPG) fluids, have resulted in practically the same leakoff coefficients. This is in contrast to previous, dynamic, leakoff tests suggesting that borate-crosslinked fluids perform better. Within the temperature range where the polymers are thermally stable (200 degrees F), there were no temperature effects after correcting for the filtrate viscosity. Under constant pressure differentials, both fluids exhibited compressible filtercake behavior with the leakoff coefficient approximately proportional to powers of 0.2 for borates and 0.17 for zirconates. When pressure was increased, imitating pumping and, then decreased, imitating closure, characterization of the pressure-dependent fluid loss using the leakoff coefficient as the descriptive parameter indicated a fluid-loss hysteresis for both fluids. parameter indicated a fluid-loss hysteresis for both fluids. During the pressure increase from 0 to 1400 psi both fluids showed a two-range response: incompressible behavior at low pressures and compressive behavior at higher pressures. In this range, the leakoff coefficient was practically constant. A comparison of the stress-sensitive properties has shown that while zirconate filtercakes exhibit decidedly viscoelastic properties, borate filtercakes are merely elastic. This can be properties, borate filtercakes are merely elastic. This can be attributed to the differences in the nature of the bonding of these polymers. Finally, filterpaper and core experiments, done with noncrosslinked fluids, have shown no filtercake-type behavior for a large range of core permeabilities, but rather a viscous flow dependent on porous medium characteristics.

Interpretation and modeling of filtercake leakoff experiments are presented. Crosslinked polymer fracturing fluids were used under static conditions. Relationships between ratios of leakoff coefficients and polymer loadings were determined. These can lead to the calculation of any value if one is obtained experimentally. It was determined that the filtercake behavior deviated from the square-root of pressure dependence for an incompressible cake. Experiments, using a constant pressure differential across the cake, indicate relationships pressure differential across the cake, indicate relationships between leakoff coefficient and pressure raised to powers approximately 0.25 or less depending on the pressure value whereas previous work reported a single proportionality for all pressures. Since pressure varies continuously throughout the pumping and closing of a fracture treatment it was imitated experimentally resulting in different pressure differentials at different times. The observed behavior is explained introducing the concept or filtercake resistance. Data interpretation for changing pressures indicates an apparent fluid-loss hysteresis, characterized by the behavior of polymer cakes subjected to changing pressures and by additional polymer cakes subjected to changing pressures and by additional cake build-up. A more fundamental interpretation is provided from the theory of viscoelasticity. The hydraulic provided from the theory of viscoelasticity. The hydraulic filtercake resistance, proportional to a rate-normalized pressure difference, leads to a stress-sensitive skin-effect, exactly analogous to similar situations in pressure transient analysis.