| Abstract |
One of the main goals while drilling a geothermal well is achieving the minimum drilling cost per unit of energy produced. Directional drilling makes it cheaper and easier to tap geothermal resource since we can have many wells on the same well pad and there is minimal environmental disturbance. Directional wells are more expensive to drill than vertical wells due to the use of down hole motors and the extra trip time incurred while trying to orient the well but are more cost effective because they give access to a greater geothermal resource from each pad. In Olkaria, we use varied combinations of the bottom hole assembly (BHA) to direct the bit to the intended path. In some formations however, holding or maintaining the desired well path is difficult and thus we are forced to change the BHA numerous times to correct the angle. To change the BHA, we need to pull out of hole and then run in hole with the desired assembly which we call a trip. Tripping is a very expensive process since more energy is consumed due to the draw works suspending the weight of the string which in turn inflates the operational costs incurred during this non-productive time. Wells drilled with very little control on the trajectory, lead to uncontrolled doglegs which cause high side forces on the string, which lead in turn to high torque, high drag and in some cases prevent the well getting to terminal depth (TD) as well as leading to drill string fatigue failures. Rotary steerable systems (RSS) allows the driller to steer the well to target with high precision. This paper seeks to illustrate the drilling challenges faced in the Olkaria field and how they might be avoided by the use of a rotary steerable system (RSS), inherently reducing the cost of drilling the well. |