Combining Pulsed Neutron Spectroscopy and LWD Resistivity for Analysis Behind Casing in Small Holes
- N. Beaumont-Smith (Shell U.K. Exploration & Production) | G.D. Webber (Shell U.K. Exploration & Production) | P.J. Shepherd (Schlumberger Data & Consulting Services) | R.S. Kalyanaraman (Schlumberger Data & Consulting Services)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- October 2003
- Document Type
- Journal Paper
- 335 - 341
- 2003. Society of Petroleum Engineers
- 1.5 Drill Bits, 3.3 Well & Reservoir Surveillance and Monitoring, 5.6.1 Open hole/cased hole log analysis, 2.4.3 Sand/Solids Control, 2.2.2 Perforating, 1.6 Drilling Operations, 3.3.1 Production Logging, 1.11 Drilling Fluids and Materials, 1.6.8 Through Tubing Rotary Drilling, 1.12.2 Logging While Drilling, 1.14 Casing and Cementing, 4.1.5 Processing Equipment, 5.1.2 Faults and Fracture Characterisation, 6.5.2 Water use, produced water discharge and disposal, 5.1.1 Exploration, Development, Structural Geology, 5.2 Reservoir Fluid Dynamics
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Formation analysis from behind casing is gaining more relevance in assessing reservoirs, not only in aging fields but also in new wells completed either without the full complement of openhole petrophysical measurements or with openhole measurements of inadequate quality. Before this technology was available, formation evaluation in freshly drilled holes was dependent on the availability of openhole log measurements. This paper discusses field examples in which reservoirs were evaluated without a full suite of openhole log measurements. Examples from two sidetracks in a mature North Sea field supported by water injection are presented. The small hole size limited openhole data acquisition to resistivity and gamma ray (GR) measurements, which were acquired using logging-while-drilling (LWD) technology.
In both cases, pulsed neutron (PN) measurements were recorded through casing to augment the existing data. Porosity and formation sigma (macroscopic thermal neutron capture cross section) measurements were obtained. Fluid saturations were computed using this porosity and the LWD resistivity. Saturations also were computed from the carbon/oxygen measurement, and these agreed with the resistivity-based answers. Lithology analysis was performed using elemental spectral capture yields and was reliable and consistent fieldwide.
The results show that, in a mature oil field, PN measurements can provide a complete lithology and saturation analysis in circumstances in which insufficient openhole petrophysical measurements are available.
Two sidetracks were drilled with 4 1/2-in. bits in the North Cormorant field, which is jointly owned by Shell U.K. Exploration and Production* and ExxonMobil Intl. Ltd. Openhole log data were limited to GR and resistivity measurements because of the restricted access to the hole. Following completion with a 3 1/2-in. liner, a 1 11/16-in. PN survey was acquired, primarily to obtain porosity but also to gather further information about fluids and lithology and formation sigma measurements. Fluid saturations were computed using the LWD resistivity and porosity from the PN tool.
Openhole resistivity coverage was incomplete, and some intervals had to be interpreted using only the PN data. In these zones, carbon and oxygen yields from the PN spectroscopy measurement were used to obtain fluid-saturation information. The carbon/oxygen results agreed with the LWD resistivity-based answers where both data sets were acquired. Such agreement was encouraging because this is a mature field supported by water injection, and there is some uncertainty about the water salinity and, hence, the resistivity-based saturations. Additionally, there are potential errors with the carbon/oxygen-derived saturations because of mud-filtrate invasion.
To use the measured sigma reliably in the future as a base log for time-lapse analysis, it is necessary to confirm whether the measurement is affected by temporary filtrate presence. The generally good agreement between the carbon/oxygen and resistivity saturations indicates that this condition is largely fulfilled.
The elemental capture yields were processed with a geochemical model to obtain dry-weight fractions of clay, quartz/feldspar/mica, and carbonates in the formation; these fractions were subsequently used in a lithology analysis. This analysis provides an appreciation of interval reservoir quality, which helps to put any changes in saturation into better context. In one well, it also resolved an anomaly caused by the presence of a hot (i.e., radioactive) sand.
Both well examples demonstrate that saturations derived from PN capture and spectroscopy data can offer a reliable alternative to an openhole resistivity-based saturation answer in a well-characterized field.
Infill Drilling Strategy and Objectives
North Cormorant is a mature northern North Sea oil field. Because of its complex structural nature, the field is strongly compartmentalized; accessing these isolated volumes requires low-cost drilling initiatives.
The principal low-cost drilling technique used in the North Cormorant field has been slimhole "through tubing rotary drilling" (TTRD)1 sidetracks. These sidetracks are drilled from an existing completed wellbore to target nearby compartmentalized blocks.
As a consequence of compartmentalization, pressure support is variable, resulting in unpredictable sweep patterns and high-pressure trapping through variable fault juxtaposition and seal.
Both sidetrack wells were drilled with 4 1/2-in. bits using TTRD. Because of the limitations of slim LWD tools at the time and instability issues that increased the lost-in-hole risk, the critical neutron and density porosity logs were to be acquired in open hole on wireline. Ultimately, both wells were cased without any openhole wireline logs because of high risk factors.
Good GR and wave-propagation resistivity data were acquired in memory while drilling. However, a tool failure resulted in the loss of data over the lowest reservoir section in Well B. Confronted with a lack of both porosity data and any indication of saturation over this interval, running a 1 11/16-in. PN tool became the only remaining option to acquire any information.
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