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ABSTRACT: In time-lapse seismics a dilation parameter (R) is used to link the change in two-way travel time to subsurface strain (compaction/subsidence). R is a measure of strain sensitivity of the vertical seismic P-wave velocity. Ultrasonic laboratory measurements of R are presented using unconsolidated glass beads and sands, compacted clay, artificial and natural sandstone, and shale. It is shown that R depends strongly on stress path, implying that in situ stress paths should be applied in laboratory investigations. R may also vary across a formation due to heterogeneities (lithology/stress field). Moreover, data on artificially cemented sandstones indicate that R as deduced from core data should be corrected for effects of coring - induced rock alteration. 1. INTRODUCTION Time-lapse ("4D") seismic measurements permit direct observations of seismic velocity changes caused by pore pressure and stress changes within and around producing petroleum reservoirs. In addition, changes in fluid saturation will influence the seismic response. However, in cases of no fluid injection to enhance production or weak water drive, the stress alteration is the main source of time-lapse response [1]. 4D seismics thus represents a tool for identification of depleted and non-depleted reservoir zones. It also has the potential to assess in a quantitative way the underlying stress changes and associated deformations (compaction and subsidence). The change in two-way seismic travel time (TWT) can in a simple manner be directly linked to underground deformation (vertical strain ez; defined as positive for compaction) and strain sensitivity of the vertical P-wave velocity (vPz), through the dilation parameter R [1, 2], defined as: (mathematical equation available in full paper) With this definition, assuming a homogeneous subsurface and homogeneous strain within it, the relative change in TWT can be written simply as: (mathematical equation available in full paper) A main justification for introducing the dilation parameter is that it can be considered as a characteristic property of the rock. If the P-wave velocity were sensitive to changes in the normal strain in the direction of propagation, and only that, this would indeed be the case. Clearly, this assumption has some relevance: vertical stretching of a rock tends to create horizontal cracks (or weaken grain contacts in the vertical direction), and vertical P-waves are primarily sensitive to cracks (and grain contacts) with that orientation. However, as will be illustrated below, the P-wave velocity is not only sensitive to the strain parallel to its propagation direction. This complicates the situation and questions the validity of the dilation parameter concept. Field observations, according to Hatchell and Bourne [1], give values of R of the order 1 - 5. The observed seismic R is reported to be larger for unloading (inflation of a reservoir, or stretching of the overburden above a depleting reservoir) than for loading (depletion of a reservoir) Strain sensitivity can also be measured directly in the laboratory on rock cores. Conventionally, only stress sensitivity has been reported in the literature, and laboratory data are most often limited to hydrostatic stress paths. However, in the Earth, be it in a depleting reservoir or in the surrounding rock masses, the stress (and strain) path deviates from hydrostatic.
- Europe > Norway (0.29)
- North America > United States (0.28)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Sandstone (0.72)
- Well Drilling > Drilling Operations > Coring, fishing (1.00)
- Reservoir Description and Dynamics > Reservoir Fluid Dynamics > Integration of geomechanics in models (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Seismic processing and interpretation (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Reservoir geomechanics (1.00)
ABSTRACT: To better handle borehole instability problems while drilling through shales, one needs access to relevant rock mechanical data on the formations, as well as an understanding of how the drilling mud interacts with the specific formation. An obstacle may however be insufficient amounts of cored shale material. Therefore, we have developed various methodologies where we utilize smaller samples to produce rock mechanical data on shales. We present various cases where we use these techniques to provide shale characteristics, and illustrate how they may be used to avoid borehole instabilities. These examples demonstrate the basic principle that small samples (mm-cm scale) can be used to characterize in particular shales, producing results that are consistent with those traditionally attained on larger, standard samples tested in triaxial cells. The tests generally provide data both faster and at a lower cost than conventional tests and, last but not least, they are sometimes your only choice when direct rock mechanical data are needed. 1. INTRODUCTION Borehole instability problems while drilling through shales are known to add substantial costs to the drilling operations. This may be related to incidents like tight hole/stuck pipe, kicks and mud losses. Fundamental control parameters for the well design are generally the mud weight, which should be balanced between the collapse and fracture pressures, and the orientation of the well with respect to stresses and rock mechanical properties (see for instance Fjær et al [1]). Additionally, drilling procedures should be optimized to achieve good hole cleaning and to minimize effects triggered by oscillations in the dynamic mud weight. In case of shales, another factor is the potential interaction between clay minerals and the drilling fluid. Traditionally, oil based muds (OBM) tend to be preferred due to their sealing capabilities towards the shale surface. However, due to environmental constraints, costs, or due to for instance barite sagging in high pressure/high temperature (HPHT) wells, water based muds (WBM) may be preferred. WBM more easily communicate with the shale pore fluid, potentially leading to loss of pore pressure support and thereby to accompanying time dependent instability problems. On the other hand, by adding various salts to the WBM, one may trigger time dependent interactions between the water phase and the shale that even enhance the stability [2, 3, 4, 5]. Such an interaction may be associated both with activity controlled osmotic processes as well as with ionic exchange effects where ions from the WBM exchange with ions in the clay mineral lattice [4, 5]. Common to such fluid induced phenomena is that their time dependence is controlled by diffusion related processes, which thereby also influence the time dependency of the stable mud weight window. In case of shales, the inherently low permeabilities result in correspondingly small diffusion constants and large time constants. Thus, a pre-requisite to better handle borehole instability problems is access to sufficient rock mechanical data on the relevant formations, as well as sufficient understanding of how the drilling mud interacts with the formation. This generally includes not only the intrinsic properties of the shale formation as such, but rather the bulk formation when for instance fractures are present since the latter may call for alternative solutions to cure the instability problem [6, 7].
- Europe > Norway (0.47)
- North America > United States > Texas (0.28)
- Geophysics > Seismic Surveying (0.68)
- Geophysics > Borehole Geophysics (0.48)
- Europe > Norway > Norwegian Sea > Halten Terrace > Block 6507/8 > Heidrun Field > Åre Formation (0.99)
- Europe > Norway > Norwegian Sea > Halten Terrace > Block 6507/8 > Heidrun Field > Tilje Formation (0.99)
- Europe > Norway > Norwegian Sea > Halten Terrace > Block 6507/8 > Heidrun Field > Ile Formation (0.99)
- (5 more...)
- Well Drilling > Wellbore Design > Wellbore integrity (1.00)
- Well Drilling > Drilling Operations (1.00)
- Well Drilling > Drilling Fluids and Materials (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization (1.00)
This paper (SPE 50982) was revised for publication from paper SPE 36854, first presented at the 1996 SPE European Petroleum Conference held in Milan, Italy, 22-24 October. Original manuscript received for review 8 November 1996. Revised manuscript received 21 May 1998. Paper peer approved 1 June 1998. Summary A continuous wave technique (CWT) for measurement of acoustic phase velocities on cuttings is presented. The equipment is particularly well suited for testing of small samples like cuttings, and measurements, even on sub-mm-thick shale cuttings, have been performed. This yields potential access to a new source of data on the drilled formation that can also be attained in quasireal time at the rig site. The prototype equipment developed is portable, fast, and easy to use. Tests have been performed both at the rig site and in the laboratory. Potential applications include estimation of mechanical properties of shales, effects of various fluids and drilling muds, estimation of seismic parameters, and estimation of pore pressure. P. 282
- Geology > Geological Subdiscipline > Geomechanics (0.67)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Mudrock > Shale (0.52)
- Well Drilling > Drilling Equipment (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Seismic processing and interpretation (1.00)
- Reservoir Description and Dynamics > Formation Evaluation & Management > Open hole/cased hole log analysis (0.70)
Abstract A system for acoustic velocity and static Young's modulus measurements on small (mm-size) shale samples is presented. The equipment has been tested on both soft and hard shales, including cuttings. Even though more experimental testing, must be carried out to confirm and improve its performance, it is demonstrated that reliable measurements can actually be performed on the small samples. Moreover, the results appear consistent with results attained from larger plugs using more conventional test methods. This technology represents a relatively fast and inexpensive method of attaining dynamic and static data from sub-surface formations. Use of cuttings and small samples can greatly improve the data availability, particularly where proper cores are not available. P. 23
- Europe > Norway (0.46)
- North America > United States (0.28)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Mudrock > Shale (0.90)
- Europe > Norway > North Sea > Northern North Sea > East Shetland Basin > PL 375 > Block 34/7 > Snorre Field > Statfjord Group (0.99)
- Europe > Norway > North Sea > Northern North Sea > East Shetland Basin > PL 375 > Block 34/7 > Snorre Field > Lunde Formation (0.99)
- Europe > Norway > North Sea > Northern North Sea > East Shetland Basin > PL 375 > Block 34/4 > Snorre Field > Statfjord Group (0.99)
- (9 more...)
- Reservoir Description and Dynamics > Formation Evaluation & Management (1.00)
- Well Drilling > Wellbore Design > Wellbore integrity (0.93)
- Reservoir Description and Dynamics > Reservoir Characterization > Seismic processing and interpretation (0.69)
Abstract A continuous wave technique, CWT, for measurement of acoustic phase velocities on cuttings is presented. The equipment is particularly well suited for testing of small samples like cuttings, and measurements even on sub-mm thick shale cuttings have been performed. This yields potential access to a new source of data on the drilled formation that can also be attained in quasi real-time at the rig-site. The prototype equipment developed is portable, fast and easy to use. Tests have been performed both at the rig-site, as well as in the laboratory. Potential applications are e.g. within estimation of mechanical properties of shales, effects of various fluids/drilling muds, estimation of seismic parameters, and estimation of pore pressure. Introduction Acoustic velocities may represent a valuable source of information on formation properties due to the interaction between the propagating wave and the medium it travels through. Traditionally, seismic data are attained prior to drilling. Sonic log data are mainly acquired after, but to some extent during, drilling. Laboratory measurements, usually at ultrasonic frequencies, can be made subsequent to drilling in intervals where successful coring has taken place. Cuttings produced during drilling represent another, potential quasi real-time source of information that can be attained at the rig-site, as well as a source of information in intervals lacking proper logs and cores. The utilization of cuttings for acoustic measurements has been limited so far, partly due to the difficulties in performing measurements on very small samples. We are only aware of one related paper on this topic, which describes a pulse transmission technique. Furthermore, the ability to perform acoustic measurements on small samples generally yields access to more material, allowing a larger variety of experimental parameters to be tested on a limited amount of material. Here we present the development and examples of use of a Continuous Wave Technique, CWT, implemented to quickly and inexpensively attain accurate acoustic phase velocities (P- and S-wave) on small samples, in particular cuttings, at ultrasonic frequencies. The CWT methodology has been patented. CWT is particularly well suited for measurements on fine-grained materials like shale, where samples of sub-mm thickness may be handled. CWT has been tested both at the rig-site and in the laboratory, including comparison between velocities as derived from both CWT and sonic logs. Potential applications of CWT include:Estimation of mechanical properties in shales from measured velocities. Such correlations have been published and may allow for a tuning of the mud weight to try to prevent borehole instabilities in shale during the drilling operation. Furthermore, the data may be utilized to improve the selection of drilling parameters. Effects of exposure to various fluids/drilling muds. Knowing that e.g. the mechanical properties of shales tend to be sensitive to exposed fluids, quantification of such effects is important when deciding mud composition during drilling. Since the samples are small, fluid effects will occur relatively fast, reducing the time required to observe fluid effects in materials of low permeability. Estimation of seismic parameters which are needed as input to seismic interpretation. For instance, velocities can be provided in intervals not logged. Furthermore, when proper samples can be found, velocity anisotropies may be determined through measurements on samples with various orientations. Estimation of pore pressure. In addition to presenting velocity measurements themselves, an application specifically related to shale-fluid interaction will be illustrated in this Paper. P. 349
- Europe > Norway (0.68)
- North America > United States (0.46)
- Well Drilling > Wellbore Design > Wellbore integrity (1.00)
- Reservoir Description and Dynamics > Reservoir Fluid Dynamics > Flow in porous media (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Seismic processing and interpretation (1.00)