Petroleum Development Oman is developing a major oil field, Haima West, to increase production levels using a new production technology not previously applied in Oman. The wells will co-produce large quantities of sand to improve productivity. The sand needs to be separated from the well fluids at the surface to protect the integrity of the production facility. No sand separation technology has been tested elsewhere in operating conditions comparable to Haima West. This paper describes the results of field tests performed on a hydrocyclone, processing the high viscosity multiphase fluid. The objectives of the tests were to evaluate the sand separation performance and determine the capacity for the Haima West conditions. In total, 100 tests were conducted and the main findings were: 1. At very low GLR (< 5 Std.m(3)/m(3)) the differential pressure should be at least 200 kPa to obtain recoveries of over 80% at a feed particle size of 200 micron (d(80) = 200 micron). 2. The recovery was improved in a step change when free water. in excess of 20% of the crude flow, was added. 3. Gas reduces the gross handling capacity for a given pressure differential over the hydrocyclone. 4. Gas, particularly at low differential pressures (< 200 kPa), has a beneficial effect on the separation performance. To the authors' knowledge, this is the first time that such complex tests have been undertaken in the field. The impact of gas and free water on the separation performance have not previously been observed and documented.