Case Study Drilling South Sudan
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Drilling an unsuccessful borehole: lessons learnt from South Sudan
In 2017, Groundwater Relief supported the International Organisation for Migration to drill two new boreholes in a Protection of Civilians (POC) camp in Bentiu, northern South Sudan, following failure of the main water supply borehole in one sector of the camp. As far as possible, the drilling campaign adhered to the Code of Practice for Cost Effective Boreholes , however a lack of appropriate equipment meant that the first borehole ultimately failed, resulting in termination of the drilling contract. This case study highlights some of the challenges of drilling in locations with poor access, limited drilling capacity, and a lack of suitable equipment for the aquifer conditions. Nevertheless, it illustrates the importance of following the key principles for cost-effective boreholes: 1. Borehole drilling and supervision is undertaken by professional drillers and consultants 2. Appropriate siting practices are used 3. The construction method is appropriate to the borehole design 4. Procurement procedures ensure contracts are awarded to qualified contractors 5. Borehole design is cost-effective, fit for purpose, and appropriate for the groundwater and aquifer conditions 6. Arrangements are made to ensure proper contract management, supervision and payment of contractors 7. High quality data is collected during borehole construction and development 8. Data is recorded, stored (ideally by a central government institution) and, where possible, made freely available 9. Boreholes are monitored regularly A more detailed description of this drilling campaign, including its successes and failures, can be found in the report compiled by Groundwater Relief .
The IOM attempted a competitive bidding process for the drilling contract however only one driller was able to respond to the tender to drill two 220m deep, 12” diameter boreholes at the POC. A detailed Bill of Quantities and Terms of Reference were included in the contract to ensure the borehole design and materials used were appropriate. This included specification of the borehole depth and diameter, casing and screen depths, gravel pack and bentonite seal, and borehole development process. Contracting Groundwater Relief to support and supervise the drilling ensured that a hydrogeologist was on-site throughout the drilling campaign to assist IOM and the local drilling contractor.
Siting, design and construction
The first borehole, which was directly replacing the failed borehole, had to be located in the existing compound to make use of the existing infrastructure, including a solar pump system. The second borehole, which was to act as a backup for the POC, was sited as far as possible from existing abstraction boreholes to minimize interference. The two new boreholes were designed based on geological information collected during previous drilling campaigns. The existing failed borehole was drilled to a depth of 126m and was screened largely within clay and silt horizons. Geological information suggested that thick beds of coarse sand and gravel were present below 156m depth, therefore the new boreholes were designed to 220m depth with screened sections between 188-218m, with flexibility to change this based on conditions encountered during drilling. The gravel pack was deemed essential to avoid or minimize previous problems with fine sediments entering the borehole. The drilling, which was unavoidably taking place in the rainy season, was initially delayed as the drilling contractor attempted to bring appropriate equipment to the site. Poor road conditions meant that their cable percussion rig couldn’t be transported to the POC and so a mud-rotary rig, which was in poor condition, had to be used instead. Drilling of the first borehole was terminated at a depth of 200m as the drillers were unable to reach the target depth of 220m with their rig. A limited-capacity mud pump was unable to remove coarser sediments from the borehole, resulting in heavy drilling muds accumulating at the base. This caused problems with installation of the casing and gravel pack. The casing was eventually successfully installed with 20m of screen positioned in medium to coarse sands between 163m and 183m depth. The gravel pack could not be installed in the correct position due to heavy drilling muds in the annulus space impeding gravel movement down the hole. During borehole development, fine sands entered and blocked the screen before a natural gravel pack could form. Initial attempts to clear the sands failed and a more aggressive well cleaning approach was attempted by the drillers, which ultimately damaged the casing. At this point IOM terminated the drilling contract.
Although ultimately unsuccessful, several lessons were learnt from this project, some of which are relevant to any drilling campaign, and some of which are particular to drilling in the unconsolidated sediments of the Umm Ruwaba Formation in the Nile Basin. • An equipment inspection should ideally be carried out as part of the procurement process to ensure the drilling company has the necessary equipment to complete the project • Even in coarse sand and gravel deposits, fine sediments can be a significant problem in unconsolidated deposits and an appropriate method has to be employed to clear them during borehole development – in this case an eductor pipe is deemed to be the best option • An appropriately placed casing, screen and gravel pack is essential for successful borehole development in unconsolidated sediments • The relationship between client, supervisor and driller can be challenging but communication is essential to ensure success • It is important that drilling contractors are held to high standards to build capacity and improve the success rate for boreholes in future For more detailed recommendations on drilling in the unconsolidated sediments in South Sudan, see the report by Groundwater Relief.