DTH hammer drilling for water wells is a drilling technique that utilizes compressed air to drive the well. The main components of this technology include DTH hammers, DTH drill bits, drill pipes, and air compressors. Among them, the DTH hammer is the core component. It uses the high-pressure power generated by compressed air to drive the internal piston and apply impact force to the drill bit, thus breaking rocks and digging water wells. If done correctly, it can significantly improve drilling efficiency.

Hole Opening

Before drilling, the DTH hammer should be inspected and lubricated with oil. If required, a test run should be conducted at the wellhead to confirm the proper functioning of the DTH hammer before initiating the drilling process. In cases where the overburden is shallow or the water table is dry and gravelly, direct utilization of a DTH hammer for hole drilling is a viable option.

Overburden drilling

A thick overburden layer and a very high water table do not apply to drilling with a DTH hammer. In scenarios where the cover layer is substantial, yet the water table is significantly deep, and only minimal surface seepage exists between the cover layer and the bedrock at 1-2 meters, or if there’s substantial surface water around 1-2 meters, with the remaining layers being dry gravel or stony soil, etc., drilling directly to the bedrock using a DTH hammer is a viable approach. During drilling, if there’s a minimal chip discharge and the expelled material appears as a moist powder, indicating water seepage at that section, it’s advisable to slow down the drilling. If necessary, air supply pipe injection or foam agent injection into the pool can be employed while increasing the frequency of “strong blowing” cycles. This approach generally helps overcome the issue. After reaching the bedrock and blowing the bottom of the hole, lift the drill. The casing should be straight down to the bedrock, and then the wellhead tightly will hit the bedrock of the DTH hammer into the well, using a diameter to the end of the method of drilling water wells.

In a different scenario, if the drilling progress is exceptionally rapid, accompanied by an increased volume of debris, it is crucial to exercise extra caution in identifying the nature of the rock debris. If the debris is determined to be sand, it becomes essential to adjust the drilling speed accordingly. It involves moderating the wind pressure, slightly elevating the rotational speed, and decreasing the frequency of “strong blowing.” This cautious approach is crucial in a sand layer, as excessive “strong blowing” in such conditions can lead to the blowing of debris into the bottom of the hole, creating a sizable void and posing the risk of a collapse that could bury the drilling operation.

Bedrock drilling

Before commencing drilling, it’s essential to introduce a small quantity of lubricating oil into the DTH hammer and carefully check whether there is mud, sand, or other debris inside each drill pipe. If necessary, blow it with the wind. If a power head drilling rig is applied, every time a drill pipe is connected, it should be blown hard with compressed air to blow away the debris inside the drill pipe before connecting it. Following the connection of the last drill pipe, continuous air supply during drilling is crucial until the rock debris at the well’s bottom is completely cleared, signaling the readiness for formal drilling.

During the drilling process, you should always observe the chip removal situation and the change of wind pressure and deal with the sudden change in time. If the wind pressure, drilling speed, and chip removal are normal during drilling, you can generally drill until you finish with a drill pipe. Before connecting the new drill pipe, we should thoroughly remove the rock chips from the bottom of the hole. It involves lifting the drill pipe approximately 200mm to utilize all the air supply for a thorough “force blowing” of the well. It is best to move the drill tool up and down several times to drain the cuttings from the bottom of the hole, then slowly stop the wind and connect the new drill pipe. Under any circumstances, when the rock drilling tool is at the bottom of the hole, the airflow must not be stopped suddenly, because this often causes the mud and sand around the bottom of the hole to easily flow back into the DTH hammer due to back pressure and prevent it from working.

In the event of a fault during the drilling process, it is advisable to slow down the rotation and exercise control over the cutting speed. It is particularly beneficial to decrease the axial pressure to mitigate the risk of breaking the carbide column or encountering a drill getting stuck.

When encountering caves or substantial fissure formations during drilling, it is recommended to employ slow rotation and maintain a low drilling speed. In the case of an abnormal drop during drilling, it’s crucial to halt the drilling process and refrain from forcing the operation.

When groundwater infiltrates during drilling, and the stratum is in a moist state, the approach depends on the thickness of the stratum. If the stratum is thin and there is sufficient air supply pressure and volume, increasing the frequency of “strong blows” can often help pass through the humid stratum. This process absorbs moisture from the rock powder in the dry stratum below. If blockages occur, proper drilling can resume.

For thicker wet strata, injecting water or foam through the air supply pipe during “force blowing” and periodically conducting thorough cleaning usually suffices, and no special treatment is generally required.

Reverse circulation drilling

In reverse circulation drilling, compressed air is fed to the DTH hammer through the annular gap of the double-walled drill pipe, with the crushed rock cuttings being exhausted from the inner hole. This configuration results in minimal resistance, leading to low losses of air supply pressure and volume. Consequently, the chip removal efficiency is excellent, with cuttings ascending to the surface in the order of being cut. This method allows for providing accurate geological samples at any given moment.

Moreover, the drilling process in reverse circulation entails minimal damage to the hole wall, making it particularly well-suited for drilling in complex formations.

In reverse circulation drilling, critical components such as the double-channel faucet, forward and reverse joints, and the separator on the rear joint of the DTH hammer play pivotal roles. Throughout the drilling process, it is essential to conduct a “forced blow” of air each time a new drill pipe is connected or before drilling is halted. This step is crucial for clearing cuttings from the drill pipe, ensuring effective removal, and ceasing the air supply.

The primary drawback of reverse circulation drilling is difficulty dealing with the isolator. If not handled properly or if it doesn’t align well with the whole wall, the effectiveness of the process is compromised. Additionally, troubleshooting can be complex, and the initial investment in drill pipes and other equipment is relatively high. The management and maintenance of the equipment also present complexities. Therefore, reverse circulation drilling should be reserved for situations where the air compressor capacity is insufficient, especially in dealing with complex formations such as fractures, sand surges, collapses, and crushings, or for drilling large diameter and deep holes. In general, it is rarely used in DTH hammer drilling.