Ultralightweight and Gas Migration Slurries: An Excellent Solution for Gas Wells

Abstract

This paper shows the excellent results obtained using ultra lightweight and gas migration slurries (ULWGMS) on wells of San Joaquin and Santa Rosa fields, located in Anaco City, Eastern Venezuela. There are several gas fields in Anaco area, with complex wells, due to their low frac gradients, pressurized water zones, depressurized gas zones, H2S and CO2 content, clay sands, and, in some cases, presence of micro fractures. These sands are also bounded by interbedded shales, with trapped gas and water, experiencing larges amount of losses of mineral oil mud, during drilling. All these characteristics contributed to unsuccessful primary cement jobs in the past, attributed to inappropriate slurry designs, with unsuitable properties to deal with all the above factors. Statistics showed that 2 out of 3 wells in this field failed due lost circulation issues, requiring remedial cementing to address gas control, and water migration. Ultra lightweight slurries were conceived, with adequate mechanical properties (flexible and tension strengths), in addition to providing H2S and CO2 resistance. The first 9.2 ppg ultra lightweight slurry was first used in 2003 in San Joaquin field, and indicated good isolation, by logs runned. Subsequently, ultra lightweight slurries with densities from 9 to 11 ppg have been applied in 18 different wells in Anaco area, showing excellent zone isolation results, demonstrated by low water cut, and good bond logs. Introduction One of the major concerns in cementing of oil and gas wells is annular gas/fluid flow migration. Over the past two decades, a myriad of different additives have been developed in order to mitigate gas/fluids invasion. Other factors must be equally considered on the planning of cementing work:Slurry Design: the recommended slurry must be able to stop gas/fluids invasion during the transtition of liquid to solid state.Hydrostatics: The total annular column hydrostatic pressure must be greater than the pore pressure of gas/fluid bearing formation to prevent gas/fluid invasion into the cement column. Besides, it must not exceed fracturing pressure of the formation to avoid losses.1Loss circulation: jobs must be designed so that frac pressure during the pumping is not exceeded. Anaco City is located at eastern Venezuela, in Anzoategui State, where the main fields are Santa Rosa, San Joaquin, Santa Ana, Guario and El Toco, as showed in Fig. 1. These fields have been produced for approximately 50 years, and their strategic importance is high, as they produce approximately 60% of total gas produced in Venezuela. Cementing operations demand special attention to achieve good zonal isolation. Reservoirs in these fields are quite complex, characterized by high and low presurized gas zones/ pockets, H2S and CO2 presence, micro fractures, lost circulation zones, and close gap between pore and fracture gradients.

Improvement and Control of Cement Slurry Formulation Resistant to the Critical Conditions HPHT

“…One of the major concerns in cementing of oil and gas wells is annular gas/fluid flow migration. Over the past two decades, a myriad of different additives have been developed in order to mitigate gas/fluids invasion [16]. The CHA is a closed system in which N 2 gas is injected into the bottom of a cement slurry at any time during its hydration (setting), and the cement's susceptibility to gas migration is determined by whether or not the N 2 Gas Injection Pressure is transmitted up through the column of cement, to the opposite (top) side of the cement sample, where the Pore Pressure is measured.If no gas migration occurs, the Pore Pressure will continue to drop during hydration.…”