Research project of Mathieu BECK and Denis GIRARDET

Evaluation of 1 and 2 dimensional electrical logging for hydrofracturing improvement. A case study for groundwater resources in hard rock aquifer of Burkina Faso, Africa.

RESUME
Le Burkina Faso est un pays soumis aux climats sahélien et soudano-sahélien et rencontre, du fait de la rareté des pluies, des difficultés à s'approvisionner en eau potable.
Les trois quarts du pays sont situés en contexte géologique de socle granitique précambrien où l'eau se concentre dans les zones fracturées sous une couche d'altérite. Les ressources en eau disponibles sont majoritairement fournies par des forages et pour améliorer leur productivité, il est possible d'employer la technique d'hydrofracturation.
Actuellement, la mise en oeuvre de l'hydrofracturation dans des projets d'hydraulique villageoise est empirique. Le packer est positionné après consultation des notes de terrain prises par le foreur (lithologie et temps d'avancement des tiges de forage) qui peuvent être assez imprécises.
Des mesures électriques en forage, diagraphies, tomographies (inversion 1D) et dilution technique ont été réalisées dans 9 forages, dont 5 avant et après hydrofracturation.
L’étude a montré que les diagraphies expéditives, peu coûteuses et pouvant facilement être construites artisanalement, permettent de déterminer précisément les fractures et sont nécessaires pour positionner de façon optimum le packer.
La tomographie et la dilution technique ont mis en évidence les effets de l’hydrofracturation, la première en terme de variation de résistivité et la seconde en terme de changement des débits statiques.
Des inversions de tomographies en forage ont également été réalisées. L’imagerie obtenue, à une dimension, n’a pas apporté beaucoup d’information.


ABSTRACT
Burkina Faso is a country subjected to sahelian and soudano-sahelian climates. Because of the scarcity of rain the country encounters difficulties of being supplied with drinking water. Three quarters of the country are located in a geological context of Precambrian granitic basement where water is concentrated in the fractured zones, under a cover of alterite (clayish zone). The water resources available are mainly provided by boreholes. In order to improve their productivity and make them exploitable, it is possible to employ hydrofracturing. Basically, this technique consists in positioning an inflatable hard rubber balloon (called packer) into the borehole to seal off a section of the well. Water is then injected at high pressure through the packer intending to open widely the fractures occurring inside the rocks. A successful hydrofracturing allows the flow to increase from a few tens of percent to more than a hundred percent. Currently in Burkina Faso, the implementation of hydrofracturing in projects of “hydraulique villageoise” is empirical. The packer is positioned after consultation of the notes taken by the driller (lithology and time of advance of the drill rods) which can be rather vague. Electrical measurements in boreholes, loggings, tomography and technical dilution were carried out in 9 boreholes (including 5 before and after hydrofracturing). The study showed that the inexpensive and easily handmade expeditious loggings make it possible to precisely determine the fractures and are necessary to position the packer in an optimum way. This work allowed the selection of the most adequate tool for a routine use.
Tomography and technical dilution highlighted the effects of hydrofracturing, the first in terms of resistivity variations and the latter in terms of static flow changes.
Electrical imaging in drilling also were carried out. The image obtained, in one dimension (1D), has not brought much information.

 

Abstract:
Groundwater prospecting is one of the major goal in Sahelian countries. In Burkina Faso, groundwater prospecting is more difficult than in other countries of West Africa because the majority of the basement rock is granitic. In such a geological context, the majority of the water available is catched inside the fractures hidden by an alterite zone (clayey zone). Geophysical methods are commonly used from the surface in order to help the hydrogeologist to implement the borehole. Even with this survey strategy, some drill holes remain wet or the yields are too low.
The Office National des Puits et Forages State Institute of Burkina Faso (ONPF) has recently decided to use the hydrofracturing technique (fig.1) in order to improve the yields when they remain between 300 and 600 l/h. This technique as been widely used (at the origin by petroleum industry) for 20 years in various situations such as mining, geotechnics, or sometimes in groundwater prospecting. Basically it consists in positioning an inflatable hard rubber balloon (called packer) into the borehole to sealed off a section of the well and injecting water at high pressure through the packer in order to intent to open widely the fractures occurring inside the rocks. The pressure within the sealed off section of the well will rise as the formation resist, and suddenly drop off if a fracture open. Note that by using one packer, only the weakest fracture would be affected. A successful hydrofracturing allows the yield to increase from few tens of percent to more than hundred percent. This technique as been tested in Burkina Faso in 1987, but it is only in 1998 that it was used widely.

Figure 1: Hydrofracturing technique

Our training period is devoted to the test of some geophysical logging techniques, (basically electrical method because they are cheap to lay out and because this material (fig.2) is easy to construct by developing countries), in order to:
i) image the borehole walls during the hydrofracturing sequence,
ii) determine which technique is able to help the hydrogeologist when positioning the packer and the casing.

Figure 2: material for electrical logging (direct current):
1: multi-electrical cable (95m)
2: logging tool
3:battery
4:resistivimeter
5:commutation box
6:multimeter

To achieve those objectives, we intent to use the following techniques:

• classical electrical logging including normals (16”, 32”, 64”) and lateral, Spontaneous Polarisation, fluid temperature and conductivity.
• 2 D electrical imaging using the most recent resistivimeter in order to produce qualitative images of the borehole resistivity. We should say that this technique is quite new in such geological context .

During the training course in Africa, we will work with the geophysical team of ONPF Institute, with Marc Descloitres, geophysicist of “Institut de Recherche pour le Développement” (IRD) French Institute at Ouagadougou, as scientific supervisor We have recently tested with him our technical approaches in a borehole in order to validate the capabilities of these type of acquisitions. Those results are presented in the figure 3 and are encouraging.
All the theoretical part of our training will be done under the supervision of Pr Dominique Chapellier, at Lausanne University, Department of Geophysics.
The results will be presented as a Poster in GEOFCAN meeting in September, Orleans, and at the end as a public presentation in February 2002, University of Lausanne.

Figure 3 : exemple of pseudosection and electrical loggings in borehole in comparison with lithology given by the ONPF. The electrical methods seem much more precise.(Manésé, April 01)

---

Addresses :

Mathieu Beck & Denis Girardet
Institut de Geophysique
Universite de Lausanne
1015 Lausanne
SUISSE

Mathieu Beck & Denis Girardet
S/c
Marc Descloitre
BP :182
Ouagadougou 01
BURKINA FASO

mathieu.beck@etu.unil.ch
denis.girardet@etu.unil.ch

 

Adresses in the South:

Marc Descloîtres
IRD
01 BP 182
Ouagadougou 01
Burkina faso
tél: 00226 30.67.37
fax: 00226 31.03.85
Il est notre responsable scientifique sur place.


Office National des Puits et des Forages
06 BP 6792
Ouagadougou 06
tél: 00226 36.19.30
fax: 00226 36.38.57
Justin Lompo (responsable prospection), Mamadou Ouédraogo, Aminou
Cissé (chefs de mission) sont les personnes avec qui l'on collabore
le plus au sein de l'ONPF et sont donc les personnes à contacter si
besoin est.