M.R. Jackson, SPE, M. Rylance, SPE and L.G. Acosta, SPE BP Exploration (Colombia) Ltd Abstract The Cusiana and Cupiagua fields are located in the Casanare region of Colombia. Current daily production (June 1997) is of the order of 185,000 stb/d, with production due to increase substantially. The reservoirs are severely compacted quartz arenites, of low porosity but with large pore throats. This makes them extremely vulnerable to formation damage by mud invasion. The extremely high compressive strength of the rock reduces perforation penetration, resulting in high skin factors. The skin factors are rate dependent due to turbulence associated with the high production rates, production of oil below the bubble point (Cusiana) and condensate banking effects (Cupiagua). The wells are costly to drill, so in order to maximise the production rates hydraulic fracturing has been applied. Post fracture well rates of up to 21 mstbpd have been achieved. At these rates non- Darcy flow in the fractures is extremely important, all the more so as problems with fracture-wellbore communication and the initiation of multiple fractures have been experienced at modest deviations, as a direct result of an extreme contrast between the two principal horizontal stresses. Wide variations in the minimum horizontal stress have also been encountered in this tectonically active area. The high Young's modulus of the rock limits the achievable fracture width and therefore proppant coverage. The paper outlines the progress made to date, and the results achieved in these economically significant but technically challenging fields. Introduction The first fracture treatment pumped by BP in Colombia was on a highly damaged exploration well, that succeeded in increasing the production rate from 1.6 mstbpd to 12.5 mstbpd plus associated gas. Following the success of this treatment it was decided to initiate a fracture programme in the Cusiana and Cupiagua fields. These fields tend to have high kh values, and therefore despite high skins can deliver reasonable production rates. However the high drilling costs, in excess of US $ 10 million per well, require high production rates to provide attractive economics. Large diameter tubing strings (usually 5.5"or 7" are run), and the challenge is to complete wells with the potential to deliver ca. 15 – 25 mstbpd. Various techniques have been tried to overcome the severe formation damage, but to date hydraulic fracturing has proven the most successful method, despite it's associated problems. Results achieved so far are summarised below. Results achieved to date As at the end of May 1997 the total number of fracture treatments pumped was 13, namely: P. 425^