Manager of the Majorca site : Philippe Pézard

The Majorca site (12,000 m2) is located at Ses Sitjoles, in the southeast of the island. It offers an aquifer in a Miocene reefal limestone setting, highly permeable, with a number of meter-scale karstic cavities. Intensive farming and irrigation in this area have led to over-exploitation of the water tables, which has caused intrusions of salt water up to 15 km from the coast, resulting in chloride pollution of the water tables. The experimental site includes a network of twelve 100-m boreholes, six of which are fully cored. The site is monitored by a team from UMR Géosciences Montpellier for:

  • In situ characterization of the site by borehole-measurement campaigns: geologic structure intersected (imaging of walls); petrophysical characterization (electrical, acoustic, natural radioactivity); and characterization of flows by hydrogeophysical methods (discharge, self-potential, hydrodispersive behavior, etc.).
  • Petrophysical and x-ray tomographic characterization of cores
  • Continuous in situ monitoring of the reservoir using new geophysical observation techniques (igeo-SER) for periodic measurements (hourly or daily) of parameters such as electrical resistivity and electrokinetic potential, also hydrodynamic parameters (Hydreka piezometers) for measuring pressure and temperature fields, and the ionic load of in situ fluids.
  • Regular in situ sampling and analysis of fluids.
  • Analysis of in situ monitoring data to study the reservoir’s response to outside stresses both natural (climatic, tectonic, pumping, etc.) and induced during experimentation (injection of a tracer or a long-duration flow).
  • Simulation and modeling of transport processes within the reservoir.

1. Scientific Goals

The Majorca Site (Baleares, Spain) was initially established under the European ALIANCE Project (FP5, 2002-2005). The site is located at Ses Sitjoles, 8 km southwest of the town of Campos, in the southeastern part of the island of Majorca. It comprises a network of twelve 100-m-deep boreholes (seven of them fully cored) on a one-hectare (100 m square) site. These boreholes intersect the Miocene reefal limestones in the southeast of the island, six km from the Mediterranean coast. In that area low recharge and over-exploitation due to agriculture have led to an intrusion of salt water up to 15 km into the island, and thus to chloride pollution of the groundwater. At Ses Sitjoles the start of the transition from fresh water (1.5 g/l) to salt water (3 g/l) lies at a depth of 60 m. The environment is totally saturated in salt water at a depth of about 75 m. The base of the fresh water corresponds to the highly heterogeneous core of the reef, the intrusion being located in the most homogeneous part, consisting mainly of talus.

The Majorca Site (Campos) is both an observation and an experimental site.

Site for observing:

  • a saline intrusion in an insular, semi-arid setting,
  • the impact of this intrusion, caused by agricultural over-exploitation.

Site for research and experiment on:

  • the heterogeneous structure of reefal formations, at scales ranging from µm (x-ray) to 100 m (depth of investigation);
  • the heterogeneous nature of flows in a reefal setting;
  • carbonate diagenesis in a saline intrusion setting.

 

Figure 1: Campos Experimental Site (SE Majorca, Spain). a) Balearic Islands. b) Outcrop at Cabo Blanco, 15 km west of the Ses Sitjoles site. c) Map of the experimental site. The cored and destructively-drilled boreholes are indicated by red stars and green circles respectively. The blue circles and triangles indicate preexisting boreholes (Pezard et al., 2008, submitted).

 

2. Monitoring carried out and principal experiments conducted on the site

In-borehole surveys have enabled an in situ characterization of the experimental site: characterization of the geologic structure intersected (imagery of the walls); petrophysical characterization of the formations encountered (electrical, acoustic, and natural radioactivity); and characterization of the flows by hydrogeophysical methods (discharge, self-potential, hydrodispersive behavior, etc.).

Surface geophysical techniques (seismics, electrical, and radar) were also implemented on the experimental site, to obtain a 3-D characterization of the reservoir’s heterogeneous structure (joint program with UPPA).

 

Figure 2: Down-hole geophysical measurements and imagery of walls in Borehole MC8. a) Optical image over a 1.5-m interval, with presence of a karstic zone at the base; b) Contents of uranium (black), thorium (blue), and potassium (red); the uranium peaks correspond to the presence of red algae; c) Total gamma-ray curve (blue) and fluid conductivity (black). The latter curve shows the transition between fresh and salt water; d) Self potential measured by the MuSET (Multi-Sensor Electrical Tool) probe (red) compared to discharge data measured with the heat-pulse flowmeter (black dot). The good correlation between these data emphasizes that the self potential is dominated by the electrokinetic potential associated with radial flows around the borehole (Pezard et al., 2008, submitted).

 

Monitoring was also conducted on the Majorca site to study the reservoir’s response to outside stresses, both natural (climatic, rainfall, etc.) and anthropic (pumping, experiments, injection of tracers, etc.). These data are needed for characterizing and modeling transfer processes within the reservoir and, especially, for studying the dynamics of the transition zone at the fresh water/salt water interface.

 

The following monitorings were carried out:

  • Pressure, temperature, and conductivity of fluids (continuous, at several levels):

Two piezometric sensors were installed in early 2008 in Borehole MC8 at depths of 60 and 70 m. They record pressure, temperature, conductivity, and pH measurements every 15 minutes.

One identical sensor was positioned from October 2006 to October 2008 in the “North Well” pumping station, at a depth of a few meters, recording the same four parameters mentioned above every two hours.

  • Fluid chemistry at seven levels, focusing on the transition zone, every three months:

This measurement was performed in Borehole MC2, with a sampler at the following depths: 40, 58, 65, 70, 75, and 85 m. This sampling is repeated on a quarterly basis in the same borehole at the same six depths, and an additional sample is taken between 65 and 70 m. These measurements will in future be carried out with the SHyFT instrument (Slimline Hydraulic Formation Tester).

  • Electrical resistivity, daily, in the borehole, focusing on the transition zone:

Resistivity measurements are recorded in Borehole MC9 every meter, at depths of 39.50 to 83.50 m. This is a daily measurement; the data are transmitted to Géosciences Montpellier via GSM.

The principal hydrodynamic experiments carried out are as follows:

  • pumping tests and injections between plugs;
  • flowmeter tests;
  • hydrodispersive tracing, using the CoFIS probe (Controlled Fluid Injection Sonde).

 

Figure 3: Resistivity measurements obtained with the electrical resistivity meter installed in MC9. Comparison with induction measurements in MC8, which show that the meter is working properly.

 

3. Joint programs

These projects currently involve TOTAL (Pau/Carbonate team), UPPA (Univ. of Pau, geophysical imagery team), CEREGE (Gilbert Camoin, for carbonate diagenesis), and the Balearic Environment Ministry (for the local setting, the coring of new wells, and the timely follow-up of certain monitoring actions).

 

4. Application of data

The measurement campaigns conducted in the field, in boreholes, and on surface provide a geologic and hydrogeologic characterization of the site at scales ranging from one millimeter to one hundred meters. Supplementary measurements, mainly petrophysical, were also made in the laboratory on available drill cores, enabling a detailed description of the various formations intersected and of their physical properties. Altogether, the data make possible a detailed understanding of the site (Jaeggi, 2007; Maria-Sube, 2008, submitted), which is indispensable for anyone who works or intends to work in this area.

Original data, measured with new underground exploration probes developed in the ALIANCE Project, are also available for the Campos site. For example, these include the hydrodynamic push-pull characterization of the reservoir (Cofis) (Gouze et al., 2008) and the description of fluid flows using self potential (Muset) (Pezard et al., 2008, submitted). These data should soon be supplemented by new in situ measurements of permeability obtained with the SHyFT probe. In addition, the installation of an automatic resistivity monitoring unit will enable the recording, over the long term, of daily logs that will make possible an accurate description of the temporal dynamics of the transition between fresh and salt water.

Besides the participation of various partners in the ALIANCE Project, a number of different teams and research organization, both French and European, are currently contributing to the acquisition and analysis of data. This has led to the joint programs mentioned earlier.

These data are being put to use in theses, articles, and new experiments. There will be very little data lacking at the end of the two theses currently under way (2010), with an intact experimental capacity in situ and the option, if need be, to conduct new drilling.

The site is accessible to all researchers in the scientific community for the development of new tools and new methods. The data assembled on this site will be progressively made available to everyone via the internet, in the database established by the observatory.

 

5. Programming:

The programming follows the completion of two theses now in progress, with an average of from four to six field investigations on the site each year.

Study of structural heterogeneity:

  • At one-meter to 100-m scale, with surface or borehole geophysical techniques (seismic, electrical, and radar);
  • compilation, and additional well-log data on porosity;
  • acquisition of new in situ permeability measurements in wells MC2 and MC10, using the SHyFT probe;
  • mid-scale study of porosity from wall-rock images;li>
  • compilation of petrophysical measurements on cores from JAEGGI (MC3 & MC5), MARIA-SUBE (MC2), and TOTAL (MC10);
  • petrophysical study of continuous x-ray image profiles in MC10 and MC11.

Studies of diagenetic processes:

  • research on older processes, using cores (MC2 and MC10), with petrophysical profiles derived from measurements on cores and x-ray tomography of specific mineralized horizons (25 m);
  • study of current natural processes, with pH profiles in the transition zone;
  • study of current processes through experiments, both on cores (MC2 and MC10) and in situ, with injection of bubbles of fresh water into the sequence now saturated with salt water.

Study of the heterogeneous nature of flows in a reefal carbonate environment

in the laboratory (focusing on reactive transport) and in situ, using the CoFIS and H2E probes.