3. APPLIED RESERVOIR GEOLOGY
General
On any hydrocarbon field, the main goal is to drain as much as possible of the in-place oil and gas, at the lowest possible cost and environmental risk. This requires detailed planning, and also thorough understanding of the geology (sedimentology, stratigraphy), petrophysical properties, and fluid contacts. Once a field has been discovered and prior to production, a so-called PDO (Plan for Development and Operations) document is written and sent to the authorities (OD) for evaluation. In that document, all the aspects of field development are addressed, including information on where producers and injectors intend to be drilled. The location of these wells is based on a geological model, and on a simulation model which is refined by iteration so as to predict optimal number of producers/injectors, and their geographical placement on the field.
Small fields never evolve beyond this early stage; ie. the field is set on produced with let say three producers and one injector, and is shut down once production from these initial wells is no longer cost beneficial.
On larger fields, it is the norm to have one or several campaigns of infill drilling. That is, evaluating where the hydrocarbons have not been swept / produced, and planning additional wells at these locations.
On very large fields such as Statfjord, Gullfaks, Oseberg, Ekofisk, etc, there is continuous activity over the course of many years, with producers and/or injectors being drilled every year. On Statfjord, the drilling rate has increased since the field has gone off plateau , with an average of 15 new wells each year at present. The idea is that as long as drilling new wells is cost beneficial and provides a satisfactory NPV (net present value) to the partnership, drilling and well intervention activities can and should be maintained.
Extracting hydrocarbons from reservoirs, both in the early phase on young fields and later on mature fields, requires focused planning in the form of three main oil / gas recovery activities. These are: 1) finding areas / pockets of remaining oil (called Targeting Remaining Oil – TRO process – on Statfjord ; 2) well planning and drilling ; 3) well interventions. Reservoir geology and geophysics, together with reservoir and production engineering, are the important disciplines involved in this work.
Targeting remaining oil
The Targeting Remaining Oil process (TRO, meaning “believe” in Norwegian) on the Statfjord Field is a yearly process. It starts with a phase in the autumn (Sept-Dec) called the Prospect Phase, where maps of all remaining oil pockets (where we believe they are…) are produced. Then comes a phase called Maturing Phase (Jan-Mar) during which the size of the prospects is evaluated. Some prospects are relatively large and interesting, others are marginal in size or have a high risk of being water-flooded. The last phase is the Drilling Project Phase, where new wells are planned roughly, in a way that ideally several prospects are penetrated by any one well. Because many prospects, large and small, exist, a great amount of different prospect combinations can be generated with these pseudo-wells. At the end, it is the well projects that have the highest NPV (cost benefit) that will make up the following year’s drilling schedule.
In the Statfjord Formation, and in particular the Raude Member, the sequence stratigraphic framework is an important tool for targeting remaining oil. Each Raude zone has its best sand development in the lower part, and it is often there that we find the most remaining oil (Fig. 4.1). Oil is also often present within the many isolated sandstone channels higher up within each sequence, but generally this oil is difficult to produce because of poor lateral communication.
Flood-front monitoring maps in Raude are produced every year for each zone. These show the areas thought to be water-flooded, gas-flooded, or containing remaining oil (Figs. 4.2 and 4.3, showing only the central part of the Statfjord Field, the A platform area). Based largely on these maps, the prospects for the Raude Member are found and outlined as polygons (Fig. 4.4). The same procedure is done for all the other Statfjord Field reservoirs. Each prospect is then documented on spreadsheets, including detailed explanation and volumetrics (amount of oil) (Fig. 4.5). Such spreadsheet are generated both at the end of the Prospect Phase, and in the course of the Maturing Phase. During the subsequent Well Project Phase, maps containing all prospects are produced, and new tentative well paths (sidetracks) are designed (Fig. 4.6).
Well planning and drilling
The wells that end up on the drilling schedule are then thoroughly planned (during the TRO process, wells are designed, but not planned to a high level of detail). Well A-13 B was drilled in June 2002, and penetrated two prospects : a Brent East Flank prospect (small rotated fault block) and a Raude prospect (Fig. 4.7, seismic cross-section). Figure 4.8 shows the Raude prospect in cross-section. As one can see, it was anticipated that oil would be found in the Raude 4 and 5 zones, whereas Raude 3 (no sand), Raude 2 (isolated channels) and Raude 1 (no sand) where not expected to be produceable. The planned A-13 B well path was also put on the sequence stratigraphic diagram for the Statfjord Formation in order to evaluate the nature of the sandstones and shales expected to be penetrated (Fig. 4.9). Nearby wells in the prospect area need also to be looked at and understood (Figs. 4.10 and 4.11). In addition to this geological and geophysical evaluation, well planning also requires thorough reservoir engineering, production engineering, and drilling / well engineering work.
During drilling, well logs of the new well are loaded almost instantly (“Real Time”) into the database. For example, immediately after the borehole assembly has penetrated the top of the Raude Member, the geologist should have the logs for Nansen and Eiriksson already spliced and entered into the database, such that an interpretation of the stratigraphy encountered so far can be made (Fig. 4.12). Once A-13 B drilling is completed, the Raude zones will be interpreted (correlated in), such that the fluid results (oil, gas, or water) on the logs can be understood with respect to the stratigraphy (Fig. 4.13). It is following this entire evaluation that the perforation strategy can be determined.
Well interventions
The drilling results in terms of hydrocarbons as read from the saturation log came in as such (Fig. 4.14) : 1) Brent East Flank prospect partly oil-filled / partly water-flooded ; 2) Raude 5 water-flooded, diverging from the expected interpretation which was that of oil-filled sands ; Raude 4 oil-filled ; Raude 3 had some sand although this was not expected, and this sand layer was oil-filled ; Raude 2 oil-filled as predicted (within presumably isolated channels) ; Raude had no net sand, as predicted.
Based on these observations, the Raude 2 sandstones were first perforated. Unfortunately, only 5 000 Sm3 of oil was recovered in the course of the month these perforations produced (these Raude 2 sands are still opened, but are believed not to contribute to production anymore because of low pressure support from the aquifer). This volume of oil is however more than was expected, as no reserves were booked in the Recommendation To Drill document for A-13 B. Afterwards, the lower Raude 3 sandstone was additionally perforated, and has since produced 130 000 Sm3 oil, which is excellent taking into consideration how thin the sand layer is (there is a sequence stratigraphic explanation for this: the lower parts of sequences are often where sandstones are most laterally continuous). These data and observations are highlighted on Figures 4.15 and 4.16. One can also get a feeling for the influence of sequence stratigraphy / sedimentology on the drilling and early production results by looking at Figure 4.17.
The plan now in terms of perforation strategy is to perforate and produce Raude 4 when oil rates will have fallen down below our cut-off level of 100 Sm3 / day (should happen before the end of 2004). After Raude 4, an unexpected oil-filled sand in Eiriksson will be perforated and hopefully produced from. Then, the encountered oil in the Brent East Flank prospect will be targeted with new perforations, likely with a plug above the Statfjord Formation.