New Standards for Oilfield Study: Sampling and Well Core Analysis in the Oilfields of Salym Petroleum Development

Ja.E. Volokitin, A.V. Khabarov, V.B. Baranov, A.A. Aniskin, G. de Broucker
Salym Petroleum Development

Introduction
Drilled out and delivered to the surface, a rock specimen or a core, is, perhaps, the only reliable source of geological information on the oil-and-gas bearing formation being studied.

Compared with the scale of the whole oilfield, the well is like a thin thread suspended in a huge room. At the same time, as regards the geological simulation of the reservoir being studied, core data are the principal foundation on which all subsequent knowledge will be based.

Thus, it is difficult to overestimate the great significance of core materials in comparison with their physical volume. Therefore, the integrity of the rock material during sampling, recovery and transportation is considered to be the primary concern in the process of collecting information on the reservoir being studied.

Unfortunately, nowadays, the application of obsolete core sampling technologies results in the permanent loss of valuable information due to the mechanical disintegration of the core and the substitution of pore fluids with drilling fluid filtrate.

As mentioned many times before, SPD is committed to the principle of making decisions on the basis of quality data. According to this rule, everything possible is done in the company to achieve 100% core recovery, the preservation of its native wettability and the comprehensive analysis of a rock material sample. SPD has analysed existing technologies and sub-contractor companies which own such technologies and decided to select one of the technologies of SibBurMash Scientific Research Enterprise.

SibBurMash Scientific Research Enterprise (SRE) owns special technologies for drilling with isolated core sampling which minimize the filtration of water-based drilling fluid (WBDF) into the core, preserving its native wettability.

Improving the Effectiveness of Production during Core Sampling
Based on the experience of SHELL in other countries, core recovery to the surface under different drilling conditions comprises on average 60-70% at 20-25 metres sampled metreage per round trip.

The integration of the worldwide experience of SHELL and SibBurMash technology and the close cooperation between the specialists of the two companies has resulted in successful core sampling from 27 wells in the Salym Group’s oilfields, beginning in 2004. The overall length of the extracted core comprised over 2 kilometres with a 98.9% average quality of core recovery (average core recovery of SibBurMash ≈94%). 82 round trips were made. 100% preservation of rock material was achieved in 68 cases.

At the beginning of this cooperation, the length of the core withdrawn per trip comprised 6-12 metres (mean value for Western Siberia), which required 4-6 trips for a full core sampling in the interval being studied (formations АС10-АС11).

With the aim of minimising costs and improving the efficiency of operations, SPD reviewed its production standards. The core sample interval per trip was increased to 36 metres (Fig 1). SibBurMash successfully managed this task, which reduced the total number of trips per well to two. In January 2008, a record-breaking figure was achieved: 48.5 metres per trip with 100% recovery, in spite of severe weather during operations – minus 38 С°.

As a result, as from the year 2004 the mechanical drilling speed with core sampling increased from 1.96 m/h to7.93 m/h. The time required for 72 m core withdrawal dropped from 6 to 2 days, making it possible to reduce the core drilling time by 50%. As can be seen from the above, SibBurMash, together with the specialists of SPD raised the level of core sampling in Western Siberia to a new height.

Health & Safety Policy
Equipment & Core Sampling Technology
The method for isolated core sampling and analysis used by SPD makes it possible to receive core material with a minimum penetration of drilling fluid filtrate. An isolating core retrieval barrel with an internal plastic tube and fluid (non-polar oil) inside an inner core recovery tube is used for this purpose (Fig 2).

Isolating oil minimizes the impact of drilling fluid filtrate on the core and plastic cases provide extra protection against mechanical damage and insulate it from the environment (Fig 3). When lifting the bottom hole assembly (BHA) to the rotary table, the core-retrieving barrel is lowered to the ramp, where plastic cases containing core samples are retrieved. Six-metre cases are marked and cut into one-metre sections.

Then, small pieces of 4-5 mm thickness are cut from the end of each section and are used for express analysis of the lithology, mineralogy and rock saturation type on the well. During the sampling of section end pieces, sections are sealed from both sides and sent to the laboratory for analysis  (Fig. 4).

Splitting the core with further photographing in daylight and ultraviolet light in the laboratory allows an approximate evaluation of the visible depth of a drilling fluid filtrate (DFF). The photograph below, Pic. 5, shot in ultra violet light shows that the depth of DFF penetration into the core does not exceed 5-10 mm.

Having said that, the saturation of the internal part of the core column is virtually unaffected by a distortion effect of technical fluids. For testing purposes, SPD carried out core sampling using a luminescent solution (soluble fluorescein), which was added to the circulation system of the drilling mud during core sampling (Pic.6).

Upon withdrawal, cylindrical samples of 2 cm in diameter were drilled out from the central part of the core and immediately preserved by wax coating to be used in the future for assessing the contamination of pore fluids by drilling fluid filtrate  (Pic.7).

Laboratory research has showed that the degree of filtrate penetration into the central part of the core comprises on average 2%, i.e. practically absent.

Core Analysis with Preserved Saturation
The high percentage of core recovery and the preservation of its natural saturation lead geological and petrophysical oilfield design to a truly new level of knowledge.

Obviously, the high degree of preservation of the rock material removed to the surface is an important component in the subsequent efficient reservoir modeling based on the results of reservoir property studies in the laboratory, lithology, mineralogy, grain-size classification and the structural characteristics of the rocks being studied.

Alongside this, the authors of this article want to highlight the researcher’s thorough work of the image core materials. Alongsidelaboratory analysis results, an important role for rock material analysis is played by the core column photographs taken in daylight and in ultra violet light. Core images loaded into the program for the processing and interpretation of geological-petrophysical data compared with logged materials allow researchers to see for themselves the whole core intersection opened by a well. The information value of the digital images of a full-size core could be compared with the outcropping of rocks in the conditions of “instantaneous” recovery from the subsurface to the surface with the preservation of subsurface saturation and, in addition, are characterised by the logging materials. Such data allow the elimination of major traditional uncertainties connected with the limited resolution characteristics of logging methods, specify the lithology of rocks, show the true thickness of formations, their textural features, saturation distribution and the presence of heterogenic collectors that are not visible for logging.

The full preservation of the rock material also eliminates the problem of tying the core to the logging data, allowing the comparison of the core and the logging characteristics of rocks throughout the entire interval with high statistical relevance  (Pic.8).

Pic.8  Shows an example of the comparison of logging materials, laboratory core analysis results and photographs of core in daylight and ultra violet light.

Below are some examples showing the key role of the core information for the resolution of various geological uncertainties:

A new type of oil saturated heterogeneous reservoir– “sandy-argillaceous conglomerate” (for the particular drilling territory). If no core data had been available, Then the chances are that such a reservoir would have been missed due to its “invisibility” to the logging methods and the standard procedures used to interpret them (Pic.9).

When opening the drill core of the Achimov formation on SPD sites by exploration wells, as a rule, there are essential problems with the interpretation of logging materials. This is because of the extremely low reservoir properties of the Achimovskaya series rocks, insufficient exploration of their petrophysical characteristics, difficulties in obtaining true figures for formation water salinity, etc.

At the same time, as is already known, even slight alterations to the calibrating parameters of the interpretation model for low-permeability reservoirs result in major changes in the evaluation of their net pay thickness and their degree of saturation. All these factors may result in serious mistakes when forecasting the productivity of such reservoirs, which are complicated from the point of view of their exploration.

When analysing the logging data of one of the exploration wells of SPD’s of Achimov formation (Pic.10), one can get an impression of the presence of significant net pay and a possible perspective of oil production, in connection with the increased values of the true resistivity of sanded formations and their probable oil saturation (Pic.10, left part).

he image of a core column in ultra violet light truly demonstrates that, from the entire, apparent net pay, barely a tenth of it is oil-saturated (Pic. 10, right part).  The rest, the majority of the sand, apparently indicate either no reservoir, or the so-called sub-reservoir which, under such capillary pressure, cannot accept oil and contains exclusively osmotic water.

Another example of the high informational content of a core with the preserved saturation is the possibility of defining of the Oil-Water-Contact (OWC) and determining various saturation types of the coeval sand bodies. Pic. 11 demonstrates an example of a estimation of OWC for the АС11.2 bed based on the materials of core images in ultra violet light. The same picture (Pic. 11) demonstrates the existence of independent oil-and-water saturated channels in the interval of the AC11.3 formation.

The preservation of natural core saturation allows direct identification of the current oil-water-saturation of reservoirs using a retortion method (Pic. 12). This approach may be successfully used for directly assessing the oil saturation of core with initial oil saturation, and most especially, for identifying the residual oil saturation of water-cut zones in situations in which assessing such values as SO based on logging data is very complicated and the results are not objective.

However, it should be remembered that the oil saturation values, received on the basis of core samples, may be slightly higher, due to the inevitable loss of pore water during recovery, transportation, processing and core analysis, conditioned by degassing and moisture loss. Having said that, this effect shows most prominently in the intervals of under-saturated reservoirs which are characterised by a high content of movable water. In the area of full oil saturation where all pore water becomes irreducible, this effect, according to authors, practically disappears.

In conclusion, we should like to demonstrate another interesting example, connected with the problems of oil saturation evaluation and the productivity of heterogenic reservoirs. When drilling a well cluster on Vadelyp oilfield, a new type of reservoir was found with anomalously low oil saturation above OWC (less than 40% based on logging). However, the results of production showed the unexpectedly low water cut values, casting doubt on the reliability of the saturation model in that zone.  However, the drilling of the well with core sampling clarified the situation immediately.

It turned out that the seemingly relatively homogeneous reservoir represented a thin layer alternation of inter-layers with various properties, which were not detected based on logging because of their limited resolving capacity (Pic.13). Less permeable formations, being located close to the clean water surface, are almost fully water-saturated under the influence of capillary imbibing (absence of glow in ultraviolet light).

Therefore, what we have here are predominantly water-saturated reservoirs located higher than the defined OWC. Fortunately, due to their bad collection properties, the water cut is not high, and for the same reason their input into the well products is not significant. At the same time, the oil  saturation of inter-layers with improved reservoir properties comprises ca 50%, based on core data, which is acceptable for this type of reservoir. The visible general low saturation is conditioned by averaging the SO of oil and water saturated inter-layers.

Summary
It is possible to identify the following main aspects of the achievements during the operations carried out on core sampling and core analysis:
»    Reduction in core-sampling time is achieved due to the increase length of the core-retrieving barrel, which resulted in a reduction in the number of trips and at the same time ensured a stable high percentage of recovery

»    The reduction in operation time directly reduces labour and financial costs, lowers injury risks and improves work safety.

The Russian subcontractor SibBurMash Scientific Research Enterprise OJSC demonstrates the quality of operations at the same level, and on some parameters, even at a higher level when compared with the world’s leading core sampling companies. Nevertheless, the experience shows that work must be continued on the improvement of core sampling procedures in order to understand the complicated structure of oilfields.
Success in the area of core data acquisition enabled log interpretation to be raised to a new level of quality. Furthermore, we should like to point out that the uncertainties in the evaluation of the saturation properties of core wells and the problems of core data tying have been eliminated and digital images of core – a valuable additional source of information on target formation, have been created.