TECHNOLOGY Corrected logs indicate lithofacies around horizontal wells

G.M. Hamada
Cairo University
Giza, Egypt

A Gulf of Suez well illustrates how to correct logs to model and identify the lithofacies characteristics and petrophysical properties of reservoir rocks crossed by a horizontal lateral.

Based on data collected from modeling the formation and logging response, it was possible to draw a cross section of the formation surrounding the well bore. This type of cross section could not have been derived from data obtained in a vertical well.

The cross section displays the formation petrophysical properties along with lateral changes in water saturation and lithology along the horizontal section.

Horizontal wells

Horizontal wells provide an excellent opportunity to define reservoir rock characteristics between vertical wells. Often, horizontal wells encounter sudden changes in lithology that may indicate formation heterogeneity or the presence of a fault.

Horizontal wells are drilled to increase productivity, to improve drainage efficiently, and to lower the cost of exploitating a reservoir. Reservoirs with thin vertical pay thickness, potential coring problems, and fractured carbonates or shales have been the dominant horizontal well targets, to date. The general drilling philosophy is to traverse a large portion of the reservoir's productive interval.1

In the Middle East, horizontal wells have overcome the problem of vertical wells producing excessive water in some reservoirs, such as the carbonate reservoirs in Turkey (Shell Co. of Turkey Ltd.), the Cretaceous sandstone in the Western Desert of Egypt (Khalda Petroleum Co.), and the Paleozoic sandstone of Southern Oman (Petroleum Development Oman). References 2 and 3 are good reviews of horizontal well applications.

Horizontal wells are being drilled more frequently throughout the world. Techniques have been or are being developed for optimizing formation evaluation and producing from horizontal wells. Formation evaluation of horizontal wells provides a potential means for better reservoir appraisal.

In a vertical well, the logging objectives are generally:

Oil/water contact determination
Lithology and porosity determination
Saturation evaluation
Reservoir barrier identification
Pressure and permeability determination.

In horizontal oil wells, these objectives no longer apply for the following reasons:

Oil/water and gas/water contacts are avoided because the well's trajectory is confined to the oil zone.
Saturation is assumed to be constant from the start of the well project.
Lithology is well known from nearby vertical wells, although the information can be misleading.

Logs in horizontal wells can identify fractures; evaluate saturation, lithology, and porosity; determine pressure and permeability; and identify heterogeneities.

Candidate reservoirs

Reservoir management teams must carefully consider the most effective direction for the horizontal trajectory across the reservoir. Consideration should also be given to not only achieving optimum production, but also for obtaining measurements needed for accurate reserve estimates.

Horizontal wells are revealing that reservoirs are not as homogeneous or isotropic as previously assumed, a conclusion supported by results of enhanced oil recovery projects carried out during the past years.2 4 5

Thin reservoirs

Horizontal wells, in thin reservoirs, contact a much larger reservoir area in comparison to vertical wells.

When the reservoir has matrix permeability, the vertical permeability is usually lower than horizontal permeability because of layering and bedding.

This factor means that the production increase is not directly proportional to the horizontal well length. In many cases, the well productivity can be improved by increasing the sinuosity of the drainhole.

In the case of Badri field, Gulf of Suez, the horizontal well has a depth tolerance of only about 10 ft, but the well bore exposure was increased through sinuosity.

The well penetrated the reservoir interval at a high 85 angle and at the end of the horizontal section the angle became inverted. This change in angle, together with the drainhole sinuosity, led to controlling gas entry from the reservoir's secondary gas cap.6

Hydraulic heterogeneous reservoirs

In hydraulic heterogeneous reservoirs, a vertical well may only intersect a low-porosity interval. However, a horizontal well will intersect reservoir rocks with a variety of permeabilities and porosities, and the best intervals can be selected for perforation.

In Sidki field, Gulf of Suez, the producing sections suffer from the severe vertical heterogeneity nature of the petrophysical properties that cause fluids to vertically segregate. This segregation leads to high GOR and low oil production.

These reservoir conditions led to drilling of a horizontal well to increase oil production and keep the gas in the reservoir.

High gas and water rates

A well's gas or water coning tendencies are reduced by the lower drawdown pressures in horizontal wells.

A short production interval in a vertical well encourages a sharp pressure drawdown around the well that causes undesired high gas/oil ratios or water cuts. However, a horizontal well with its longer producing section reduces drawdown pressure and, therefore, decreases coning problems.

A lower drawdown also increases ultimate oil recovery because it reduces the risk of uneven depletion and improves the sweep across the reservoir.

In El-Salam and Hayat oil fields in the Western Desert of Egypt, vertical wells suffer from high water rates. Horizontal wells, however, have significantly reduced water production and increased oil productivity indices by a factor of 10, compared with the vertical wells in the fields.7

Correcting logs

Logging tools were originally designed to measure formation properties crossed by vertical well bores. But in horizontal wells, layers are not crossed in a continuous progression.

Depending on their measurement method, individual tools respond in a variety of ways in a horizontal well. To make an objective interpretation, one has to understand these different responses.

One of the most important aspects of interpreting horizontal wells is to prepare an appropriate logging program. This requires a clear understanding of the well's objectives and target zones and adoption of a modeling approach.3 8

In the example case, the goal was to identify the extent of the sand in the horizontal section and to assess the rock characteristics for a horizontal well in the Gulf of Suez. Fig. 1 [63587 bytes] shows the recorded logs, after borehole corrections.

If the horizontal section had stayed in the sand reservoir, one could expect the logs to have nearly consistent behavior. But the logging data indicate that lithology is changing along the traverse of the hole. This implies that the well bore trajectory is moving in and out of a sand-shale parallel interface.

Resistivity modeling

The primary task is to determine both the position of the horizontal well bore in relation to the bed boundary and the true resistivities and thickness of the strata.

In this case, individual layer resistivity values were estimated from the vertical section of the hole. Following that, the response functions of the gamma ray and neutron logs were used to estimate the distance of the well bore to the bed boundary interface. With this information, the model iteration proceeded.

Fig. 2 [80795 bytes] illustrates construction of the model for a horizontal section at 6,231 ft with 10 response positions above and below the boundary.

The tool position whose modeled log response matches most accurately with the actual resistivity logging data shown on the formation model is taken as the correct one. In this case, the tool positions -5 and +3 were taken as accurate tool positions.

Clearly, several solutions are possible. The tool response is affected by the resistivity above the boundary, the resistivity below the boundary, the distance of the well to the boundary, and the formation thickness hosting the well section.

Nuclear log

Interesting nuclear tool response characteristics also are shown in this well. Fig. 3 [84108 bytes] shows modeling responses of the gamma ray, neutron, and density logs for a shale-sand sequence. It is known that the gamma ray tool has no azimuthal focus and responds to beds above and below, and on the side of the well while neutron and density tools are effectively focused downward.

As seen in Fig. 3a [84108 bytes] and based on this fact, a shale bed crossing the well bore from above will be detected by the gamma ray tool before either of the downward-reading density and neutron logs.

If the shale bed crosses from below, however, all three of the nuclear tools will see it simultaneously (Fig. 3b [84108 bytes]).

Now, neutron and density tools can see the transition much earlier than in Fig. 3a [84108 bytes].

This model's response depicts that by studying the readings of the gamma ray and neutron-density logs, it is easy to estimate whether the layering interface is approaching from the top or the bottom.

Rock characterization

Information from the resistivity model and the nuclear tool response models were combined together for a clear idea about resistivity and lithology distribution around the boundary. This combined information, together with the recorded log data, are analyzed and interpreted in Fig. 4 [77596 bytes].

This figure illustrates the well trajectory through the beds of various resistivities. The interpretation chosen is based on the neutron-density and gamma ray responses at the boundaries 7,375 and 7,580 ft.

Considering Fig. 3 [84108 bytes] and going from the bottom of the well to the surface, it appears that the sand is approaching from the bottom at 7,375 ft while shale is approaching from the top at 7,580 ft. At depths 7,750 ft and 7,850 ft there is sudden change in lithology and boundary which could be attributed to a local fault affecting the lateral facies characteristics.

The inspection of log data in Fig. 1 [63587 bytes] shows that there are two types of sands. The sands below 7,790 ft have higher resistivity readings and lower gamma ray readings than the sands above this depth.

Petrophysical properties were evaluated with the ELAN program that includes the optional quartz-illite-oil-water model, and resistivity (ILM and ILD ), neutron-density and gamma tools responses, except below 7,880 ft, where the density log readings are not available.

The final petrophysical evaluation of the horizontal section in this well is shown in Fig. 5 [75643 bytes] using resistivities predicted by the model results and nuclear tool readings.

The sharp changes in resistivities cause some sharp changes in water saturation. As expected, the two sands below 7,790 ft have better reservoir characteristics than the two sands above.

The reservoir quality diagnosed in this example was confirmed by a production test that resulted in a moderate flow rate.

The well was eventually also completed further uphole in the vertical section.

References

1. Bigelow, E.W., "A New frontier; log interpretation in horizontal well," 33rd SPWLA Annual Logging Symposium, June 1992.

2. Nurmi, R.D., Wiltse, E., and Sapru, A, "Middle East reservoir characterization improved by data from horizontal wells," SPE Paper No. 29816, 9th Middle East Oil Show and Conference, Bahrain, March 1995.

3. Giannesimi, J.F., "Horizontal wells: an overview," Oapec-IFP Workshop on Horizontal Drilling, June 1992.

4. Giger, F.M., Combe, J., and Reiss, L.H., "L'Interet du forage horizontal pour l'exploitation de gisements d'hydrocarbures," Revue de l'IFP, Vol. 38, No 2, 1983, pp. 324-50.

5. Singer, J.M., "An Example of log interpretation in horizontal wells," The Log Analyst, Vol. 35, No. 2, 1992, pp. 85-95.

6. Ezzat, A., and Hassan, A., "Use of an advanced technique in reservoir management and it's application in Gupco's first horizontal well," Petroleum Exploration and Production Confer ence(EGPC), November 1994.

7. El-Refai, E., "Application of the two horizontal wells in Egypt," Oapec-IFP Workshop on Horizontal Drilling, June 1992.

8. Chadrac, J.L., and Wittrisch, L., "Well logging and formation evaluation in horizontal wells," Oapec-IFP Workshop on Horizontal Drilling, June 1992.