Innovations help exploit mature Trinidad land fields

Stanley R. Wharton
Amoco Trinidad Oil Co.
Houston

John Keens-DumasPetrotrin
Pointe-a-Pierre,Trinidad & Tobago

The Vessigny/Guapo/ Parrylands fields in southern Trinidad are believed to contain substantial reserves of heavy oil in deltaic sands of Cruse and Forest formation of Pliocene age.

These fields, which produced over 57 million bbl of oil (1908 to present) have been subjected to an integrated study to ascertain exploitation potential of the remaining reserves.

Wells produced from two anticlines in the study area have recorded individual initial production rates of over 8,000 b/d of oil during the exploration phase (one well produced more than 1 million bbl of oil), but production linked to natural decline has achieved rates of much less today (less than 40 b/d pumping production).

With low productive rates and the concommitant low rates of return on investment, many potential areas were deemed uneconomic at oil prices of $14/bbl.

This study, which comprised 5,000 acres, involved the application of sequence stratigraphy, 2D seismic, well log data, and biostratigraphy, combined with the application of improved completion techniques for producing the new wells.

Corporate organizational changes helped make the study possible. The strategic merging in 1993 of Trinidad and Tobago's two state oil companies, Trintopec from former Trinidad Tesoro and Trintoc from former Texaco Trinidad Inc., was significant in that greater acreage for exploration was available across former lease boundaries for accurately defining key reservoir trends.

A successful drilling program supported by a new industry business initiative between the public and private sectors showed early benefits in the post-merger period. Sequence stratigraphy was the mapping technique introduced in the fields in this exploitation phase to selectively target the objective reservoirs by accurately delineating sand trends across the field area.

Geology, stratigraphy

The surface geology is defined by young Pliocene Morne L'Enfer sands which crop out in the central section of the study area with outcrop patterns dictated by post Pliocene compression, uplift, and erosion (Fig. 1 [38224 bytes], Fig. 2 [50285 bytes]).

The area appears to have experienced greatest uplift in the northeast, where older Cruse sands of Early Pliocene age crop out at the surface defined by post Pliocene anticlinal and synclinal structures. NW-SE and NE-SW fault sets dissect the structure.

Farther to the northeast Pre-Pliocene rocks crop out at the surface along a structural high. In the north the Nariva sands of Early Miocene age have been exploited as both primary and secondary targets along a trend which is adjacent to the well known Brighton fields formerly of Texaco Trinidad.

A composite type log was subdivided into major parasequence sets to define a stratigraphic framework (Fig. 3 [31775 bytes]). The parasequence sets above and below a Maximum Flooding Surface Lower Forest (MFS-LF) marker were individually mapped to define the trends of the main reservoirs which were deposited in a middle neritic to inner neritic environment.

Each parasequence set was found to show a preferred orientation with a general SW-NE progradation which was used to determine the trend of the play to the northeast. The exploitation strategy was developed for similar trending targets in both Cruse and younger Forest reservoirs.

Seismic data

The seismic data (2D) from the most recent survey of the Southern Basin Consortium were used in this study. Key lines over the area showed reasonably good resolution from 100 ms down to greater than approximately 3,000 ms.

Integration of the sequence stratigraphic data with the seismic data revealed a clear anticlinal/synclinal structure dominated by the Guapo syncline (Fig. 2) and flanked by the equally tight anticlines that were truncated by NE-SW fault sets. The base of the Cruse deltaic sequence was clearly defined by a sequence boundary according to biostratigraphic evaluation.

Incorporation of the seismic data with well data showed good correlation with the surface geology data, with the greatest thickening of the deposits occurring at the syncline, the area of exploitation interest. Structurally, the new development targets were located mainly within the Guapo syncline in comparison to the adjacent anticlines.

Exploitation technique

Since the merger of the state companies and as a result of the increased production in a well which was drilled in 1991, renewed interest in the acreage led to limited outstep development drilling within the Guapo syncline.

Production from the key well located within the syncline, was realized from the combined transgressive systems tract of the Forest formation and highstand systems tract of the Cruse formation in what was regarded as a high producing well for the area (Fig. 3 [31775 bytes], Fig. 4 [30819 bytes]). At peak production, the flowing productive rates of the well attained a capacity of over 180 b/d of 14.5° gravity oil in 125 ft pay sands with sustained FTHP averaging 600 psi. Total depth of the well was 2,100 ft subsea.

This key well was not immune from sand entry problems in the early productive phase. This was later remedied by acidization and recompletion with a proprietary resin-coated sand technique from Halliburton (Fig. 4). These remedial techniques enabled the productive zones to produce satisfactorily at almost constant reservoir pressures for over 4 years.

The well developed a production profile that showed significant deviation from the normal decline rate models used for wells in the field of 55%, 35%, 20%, and 10%/year for the first 4 years of production. The challenge presented to the reservoir team was to develop further drilling prospects mainly in the northeast to delineate the eastern lateral extent of the potential productive reservoirs.

Exploitation strategy

The interest in the performance of the key well and its strategic location adjacent to open acreage within the tight Guapo syncline (Fig. 2) made this area a key play for exploration.

The nearest well to the northeast (approximately 2,500 ft or five well spacing units away) was drilled overbalanced with 115 pcf oil base mud (OBM) in the "Parasequence Set C" (PSC) reservoirs and completed with 120 pcf OBM. This well did not produce. The well was used in the field evaluation to predict the northeast extension of the PSC unit to the southeast on the anticline limb.

Another well to the southeast encountered only thin interbedded shale/sand parasequences believed to be the edge of the transgressive systems tract of the Lower Forest and marked the southern limit of the PSC depositional unit.

Phase I-1994

Exploitation strategy was developed in two phases (Fig. 5 [47253 bytes]).

Phase I exploitation involved an outstep drilling program within the syncline to test the northeastward limit of the productive sands found in the key well. Three wells were successfully drilled to target mainly the PSC sands and were completed using conventional techniques.

The data from the new wells were then incorporated with fieldwide data to develop new prospects in other parts of the field. These wells achieved similar production rates and showed similar reservoir trends as were predicted for the prograding sequences (Table 1 [29643 bytes]).

The initial evaluation showed the PSC and "Parasequence Set A" (PSA) reservoirs to be the most prospective in the area. Depositionally, the PSC and PSA sands in the upper part of the highstand systems tract of the Cruse formation showed better reservoir development and provided the majority of targets in the area under study. Comparatively, the PSA sands showed hydrocarbons with 20.5° API oil while the PSC reservoirs had 14° API oils. Both reservoirs were solution gas drive reservoirs.

Phase II-1995-96

Phase II exploitation was developed as a more comprehensive program in which several parasequence sets were mapped fieldwide and depositional patterns were identified across the field.

Several multiple targets in the Cruse formation were developed, and nine wells were drilled to optimize recovery of the millions of barrels of recoverable oil in place.

Targets were developed for the PSA and "Parasequence Set AA" (PSAA) sands along with the PSC and Forest B as double objectives.

The second exploration phase proved to be successful, and an under-developed PSA' channel sand (Fig. 3) located on the south flank of the Vance River anticline was considered to be a new discovery in Well No. 7 and later in Well No. 11 in the west (approximately 4,330 ft away). This sand was previously not mapped in the area.

In the latter area reservoir engineering assessments of the Cruse reservoirs for exploitation had advised that the area was depleted, and further drilling was not recommended. However, after the application of the sequence stratigraphic framework, the PSB was found to be undepleted and prospective. Well No. 11 drilled and completed in the PSA, PSA', and PSB sands recorded flowing rates greater than 250 b/d of oil sustained over a 30 day period.

The stratigraphic framework identified a laterally extensive basinal marker, the Maximum Flooding Surface Lower Forest (MFS-LF), as a key marker above the highstand systems tract of the Cruse formation and a sequence boundary separating the Cruse formation from the overlying Forest formation. The MFS-LF marker provided a reliable chronostratigraphic datum in the understanding of the underlying sequence boundary and the further delineation of the reservoirs.

Drilling arrangement

The renewed interest in exploitation presented management of the state company with a new challenge to acquire the required capital to finance the drilling projects during a period of low investment capital and low to non-existent drilling activity.

Alliance was sought and developed with private sector interest who later became the prime contractors for the subsequent projects. The prime contractors were responsible for the initial drilling and completion designs for the wells, after which the well became under the jurisdiction of the state company.

This was a unique business arrangement for the merged state companies, and it was later known as Externally Financed Packages or EFPs.

Improvements

All completions were single selective; however, wells in Phase II were completed with a different technique from those of Phase I.

New wells utilized TCP (45/8 in.) and were perforated underbalanced at six shots/ft in 75 pcf brine. This design differed from the Phase I wells which utilized conventional 4 in HSC with perforations shot at four shots/ft.

Phase II wells drilled within the syncline, were completed underbalanced and showed greater initial productive capacities similar to the key well (Table 1). These wells had fairly sustained flowing production and fairly stable pressure profiles over 700 psi.

One of the major obstacles affecting the sustained production, however, was the scourge of sand entry mainly in the wells completed in the more unconsolidated PSA unit. Over 90% of the wells were successfully drilled both within the syncline and on the downplunge end of the anticlines. The predicted NE-SW trend of the primary targets were delimited and perforated for completion.

Initial production rates for the new wells were comparatively high and initially exceeded the holding capacity of the production facilities in this semi-remote location. Cumulative oil production per well rose to a maximum of over 50,000 bbl of oil (Table 1) in one of the trouble-free wells.

Prospects remain

After the completion of Phase II drilling, which partially identified the lateral extent of the key reservoirs, the areas immediately to the northeast became prospective for both PSC and PSA sands.

Seismic data in these areas reveal a series of smaller tight anticlines and synclines paralleling the existing structure and cut by a series of NE-SW and NW-SE trending faults.

Substantial oil remains to be discovered in these adjacent areas, and in 1997, two additional development wells from this study are expected to be drilled for Cruse reservoirs.

There remain, however, the needs to develop innovative techniques to arrest the scourge of sand entry and better manage reservoir performance. Thus far, the resin coated sand control technique has had limited success, the most successful being the key well that was used to develop Phases I and II of the exploitation program.

Benefits listed

• Integration of the data sets proved the existence of several prospective areas which when drilled contributed over 200,000 bbl of oil over 2 years. Production was interspersed with intermittent sand entry problems in producing wells.

• Application of sequence stratigraphy enabled more accurate identification of targets which had greater than 95% drilling accuracy.

• A new business ar- rangement/alliance had developed between the private sector and the state company.

• New prospective areas hitherto undeveloped were generated for outstep drilling and semi-exploratory drill- ing.

Acknowledgments

The authors would like to thank Petrotrin and Amoco Trinidad Oil Co. for allowing the presentation of this information. The authors thank all persons who have made a contribution to this article and particularly those responsible for the construction of diagrams and preparation of the manuscript.

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