Ross Field, UK sector, North Sea, showing locations of the horizontal wells. The two wells discussed in the text are indicated.
Geosteering of horizontal wells is usually achieved by the integration of biostratigraphy, measurement while drilling (MWD) and geological evaluation of ditch cuttings. This combination is comp-romised if the biostratigraphic component of the geosteering toolkit lacks resolution (for example, in red-bed continental sequences), and by the use of polychrystalline diamond compact (PDC) bits, which causes severe deterioration in sample quality. Such circumstances require the appli-cation of alternative, less conventional geo-steering methods
Heavy mineral analysis (HMA) is one possible approach that can be applied to sandstone reservoirs, since heavy minerals are ubiquitous in sandstones and are frequently used in reservoir correlation. The technique involves optical microscopic analysis of heavy mineral residues from ditch cuttings at the well site, with data acquisition taking approximately two hours from receipt of sample at the surface. Wellsite HMA was pioneered on appraisal wells in the Clair Field, west of Shetland, in 1996-1997, but its first application in a field development program was on the Ross Field in the North Sea.
The main reservoir in the Ross Field (Moray Firth, North Sea) consists of shoreface sandstones of the Ross Formation, with significant reserves in the underlying alluvial/fluvial Parry Formation (both Upper Jurassic). The Ross Formation, which was deposited during progressive transgression, is informally subdivided into four units (R1, R2, R3 and R4 in ascending order). Each unit is approximately 20 ft thick, and represents a basal transgressive event followed by shoreface progradation.
Reservoir quality in the Ross Formation is strongly dependent on depositional facies, the best quality being associated with the more proximal marine facies (middle/upper shoreface) and the poorest quality with the most distal (offshore) facies. The distribution of good quality reservoir across the field is therefore dependent both on stratigraphy and paleogeographic setting.
Exploitation of the Ross Field requires drilling horizontal wells that target relatively thin pay zones. Since biostratigraphy lacks sufficient resolution to distinguish R1, R2, R3 and R4, HMA was considered to have greater potential for geosteering in this context.
Prior to its use at the well site, the applicability of HMA was investigated by analyzing existing cores of the Ross Formation from vertical or sub-vertical offset wells. The studies showed marked downhole variations in a number of key parameters, including overall detrital heavy mineral recovery, mineral ratios, and mineral grain attributes. These variations provided confidence that the method would have sufficient resolution to identify and distinguish the various stratigraphic units during drilling of horizontal wells. The most useful parameters proved to be:
- Heavy mineral recovery, which is very low in distal offshore sediments, but increases with proximity to the paleoshoreline. The decrease in recovery in the more distal sediment is due to the reduction in clastic sediment supply. Recovery also tends to be relatively low in paralic sediments (Parry Formation and R1 coastal plain facies).
- The zircon:tourmaline ratio (ZTi), which varies on a high-frequency basis throughout both Ross and Parry due to a combination of changes in sediment provenance and hydrodynamic conditions during deposition.
- The apatite:tourmaline ratio (ATi), which reflects the extent to which sediment has been weathered during the sedimentation cycle. ATi is high throughout in the marine parts of the Ross Formation (R1-R4) but very low in paralic sediments (Parry and R1 coastal plain facies)
- The abundance of euhedral and brown zoned zircons, which are indicators of sediment derived directly from the Ross granite of the adjacent hinterland. Such zircons are scarce in the Ross Formation but are common in the Parry.
To date, HMA has been used to geosteer nine horizontal wells on the Ross Field (13/29a-A3Y, 13/29a-A4, 13/28a-B2, 13/28a-B3, 13/28a- C1Z, 13/28a-C2, 13/29a-D1, 13/29a-D2 and 13/ 29a-E1). HMA is particularly effective in the Ross Field, since penetration rates (ROP) are relatively slow (typically 10-20 ft/hour), and the logging tools are placed some considerable distance behind the bit.
For example, the gamma ray tool is 54 ft behind the bit. At an ROP of 15 ft/hour, HMA can give an indication of formation change some two hours before the gamma ray log, taking into account cuttings lag time and heavy mineral extraction. Heavy mineral data were used in the decision-making process in a variety of situations, including identification of the 9 5/8 in. casing point, deciding on TD, and whether to maintain angle, to steer up or to steer down.
A typical example of the application of HMA as a geosteering tool for the Ross Field development is well 13/29a-A4. This well targeted the R3 unit, which has the best reservoir quality in this part of the Ross Field.
Heavy mineral data provide a clear distinction between R3 and the overlying non-reservoir R4 unit, and also differentiate the better-quality upper R3 from the poorer-quality lower R3. R4 is distinctive in having a combination of very low recovery and low ZTi. The upper R3 has high recovery and high ZTi, whereas the lower R3 has high recovery and low ZTi.
Consequently, when HMA indicated that the well track had penetrated lower R3 (for example, at 14230 ft and subsequently at 15710 ft), a decision was made to build angle to enable the well track to re-enter better-quality upper R3. Similarly, when HMA indicated that the well track had penetrated R4 (at 15140 ft and subsequently at 16,390 ft), a decision to drop angle was taken. This was successfully achieved after the first penetration of R4 (15,140-15,440 ft).
However, after the second R4 section (16,390-16,500 ft), R3 sediment was encountered only until 16,575 ft, at which point penetration of R4 was renewed. This indicated that it was no longer possible to steer the well, and since the target of drilling 2,500 ft of high-quality reservoir had already been achieved, TD was called.
Geological interpretation of the 13/29a-A4 well trajectory, showing the extensive penetration of good-quality upper R3 reservoir sandstone.
An additional application of HMA in the Ross Field is the identification of the 9 5/8-in. casing point. Since the overlying Heather Formation is frequently unstable and requires a much higher mud weight than required to drill the reservoir, it is important that the casing point is placed in the relatively thin R4 unit.
The R4 can be differentiated from the Heather Formation on the basis of a small increase in heavy mineral recovery and by the appearance of glauconite, as well as its typical high-gamma signature. In one well on the Ross Field, there was an MWD failure during drilling of the lower Heather.
This could have necessitated making a trip to change the bottomhole assembly, but the casing point was picked solely on the basis of the heavy mineral data and geological evaluation of ditch cuttings, thereby achieving significant cost saving.
In many of the horizontal wells on Ross, total depth (TD) is called either because the well achieved its target or because mechanical reasons made it impossible to steer adequately.
However, in the water-injector well 13/29a-E1, after drilling over 600 ft of good-quality R2 and R3 sediment, HMA indicated that the well had penetrated an upthrown basement block. In view of uncertainty over the precise throw on the fault, it was unclear how long it would take to drill through the basement back into the reservoir. Since the R2 and R3 section that had been drilled was deemed to be sufficient for the purposes of water injection, it was decided to terminate drilling at this point.
As a result of its application on the Ross Field, heavy mineral analysis now has a positive track record for geosteering horizontal wells during a field development program. The technique can therefore be genuinely considered as an alternative geosteering tool in circumstances where biostratigraphic methods have inadequate resolution.
Talisman partners, LASMO North Sea PLC, Intrepid Energy North Sea Limited, Kerr-McGee North Sea (U.K) Limited, MOC Exploration (U.K.) Limited granted permission to publish this article.